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Excellent question. Actually the answer is interrelated among the three aircraft due to their differing capabilities, differing design maturity levels, key decision points in the manufacturing and deployment process, and - of course, the exigencies of war and their performance in training and combat. All three aircraft were generally successful in use, but their applications, achievements, and reputations varied. Many factors influenced how each was designed, tested, assembled, and used - including before, during, and after the war.The first of these - Boeing B-17 - was given its basic design beginning in August 1934. It was a “direct aerodynamic and structural combination” of the Model 247 airliner and the Model 294 experimental extra-heavy bomber, then designed but unbuilt as the XB-15, and given Boeing Model number 299. It was deliberately designed to a medium bomber specification but given four engines instead of the traditional two - this because Boeing knew competition had access to the same powerplants; therefore a performance edge might be obtained by having more power on a similar-sized airframe.The Model 299 prototype emerged as an elegant emblem of the art deco style of design - simultaneously futuristically beautiful and yet practical to use. A Seattle journalist declared the prototype to be a “veritable flying fortress” - an alliterative name that Boeing was quick to trademark. The new aircraft was unlike anything seen before - quite large for its day, yet very sleek, modern, and free of any material excess. In August 1935 the new aircraft set speed records flying to the Army Air Corps fly-off competition in Dayton, Ohio. It was two hours early and embarrassed officials who were late in meeting it.Despite this success, the road ahead was rocky for the Boeing aircraft. The prototype famously crashed when elevator gust locks were left in place on takeoff. This incident highlighted the need for pilot checklists which of course are now standard in aviation. The Air Corps had much to learn about not only the aircraft, but the flight envelope it opened - early 299 Models were faster than existing Air Corps fighters. And - the plane exacerbated debate within the Army about the role of bomber aircraft and the controversial need for a more independent air service. In this environment Boeing was awarded a contract for only 13 service test models of the new airplane in January 1936, which was now officially designated ‘B-17.’As the Air Corps began to take delivery of these early B-17s more trouble arose: pilots unfamiliar with so large and sophisticated an aircraft skidded one on its nose (through misapplied brakes) and created other mishaps. Another was inadvertently flown into a thunderstorm and spun out of control until the pilots could recover and land safely. This aircraft happened to be instrumented and strain gauges recorded stresses in excess of design limits. As a result a static test airframe was converted into a test mule for new turbosupercharger technology under development at GE. These interrelated developments would prove to have long-term implications for both the B-17 and the Air Corps.Equipped with the new superchargers, B-17s entered low-volume production and progressed through three design iterations during the late 1930s. During this time the aircraft made many headlines - one of which resulted in controversy after the Air Corps sent three B-17s to meet the Italian ocean liner Rex 600 miles offshore. The Navy - always conscious of its turf - objected strongly and forced an agreement keeping Army aircraft within 100 miles of the U.S. coast. In retrospect this agreement appears anachronistic at best but provides insight into the mentality prevailing at the time. The B-17 was breaking new ground not only in performance but also in policy. The key policy - one that would shape the course of the air war - was that of daylight precision bombing. This concept would evolve into a hard-edged doctrine that the Army Air Forces would pursue with great determination.The Consolidated B-24 was a much later design. It was originally considered to be a next-generation bomber compared to the B-17 and featured a new and more efficient airfoil concept designed by aerodynamicist David R. Davis, known as the ‘Davis wing’ which maintained laminar flow over more surface area compared to conventional airfoils, reducing drag. The new bomber was related to a flying boat design, the model 31, which had the Davis wing and a twin fin and rudder layout - both B-24 characteristics. Inherited also from the flying boat predecessor was the shoulder-mounted wing layout which required long main gear legs and resulted in relatively low fuselage ground clearance. The new high-aspect ratio wing profile conferred lower drag and longer range, particularly at low- to medium altitudes.Although design work on the B-24 began in January 1939, after the B-17 had been in service for over two years, interestingly the B-24 was sized similar to the B-17 and used similar power - Pratt and Whitney R-1830 radials compared to Wright R-1820s. Each engine was in the 1000–1200 HP class. Like the B-17, the B-24 was unpressurized. However, it did feature tri-cycle landing gear - then a relatively new idea - making aircraft ground handling somewhat easier by comparison. The B-24 also had a larger bomb bay, not being restricted to the ‘medium bomber’ bay of the B-17.The B-24 benefitted from the pioneering work done by the Air Corps with the B-17. Whereas the B-17 had to be fought for at every step of the way, by the time the B-24 was in prospect, it was becoming clear that ‘heavy bombers’ would be required in any war scenario. By the time the XB-24 flew at the end of December, 1939, the war in Europe was well underway and the environment for purchasing new bomber aircraft was significantly different compared to prior years. In this time the U.S. military was both detached from, and yet intimately aware of, the impending world crisis. Contracts for new aircraft, new aircraft plants, and new airfields were let. And precursors to Lend Lease - sale of U.S. military aircraft to Britain and France - were underway. The XB-24 attracted interest from several quarters.The performance expectation for the B-24 was such that it was thought to be superior to the B-17 - and compared to early B-17 models in some respects it was, particularly when comparing range-payload between early small fin B-17s and early production B-24s. Based on these somewhat theoretical analyses, planners began working on a massive scheme to produce the B-24 in quantity, which would ultimately include five manufacturers - including the famous Ford Willow Run plant, which ultimately produced a bomber an hour, around the clock. The B-24 was destined to be the most-produced heavy bomber of the war, and also the most-produced American combat aircraft of any type.Meantime, in May 1940, the Air Corps had asked Boeing to redesign the B-17 to accommodate gun turrets and other upgrades. The result was a completely new aircraft behind the wing. As if aided by a magic wand, Boeing turned an excellent if somewhat outmoded bomber into a real thoroughbred. Several areas were addressed simultaneously: flying stability, structural integrity, and defensive armament capabilities. Now the Air Corps had two new heavy bomber designs in hand to fight the war that would come. The new B-17 version would prove to have outstanding development potential, and three plants were dedicated to producing it. The “Arsenal of Democracy” was gearing up for war.Off the military stage, Boeing had developed an airliner version of the Model 299 - the Model 307 - that incorporated cabin pressurization in an airliner for the first time in history. This aircraft - the Stratoliner - was marketed as the “1940 Airliner” but was never fully established due the war. However, the concept formed the basis for pressurized bomber studies that evolved through several designs - Model 334; Model 341; and finally Model 345 - as the war first swept through Europe and Japan continued its assault in Asia. The Air Corps was interested but from a distance since there was no official specification for the aircraft. It was Charles Lindbergh - often derided for his perspectives on Nazi Germany - who pushed the USAAF into official sponsorship of the pressurized bomber design. Boeing was now clearly in the lead in the heavy bomber business, and this new bomber was given the ‘B-29’ designation.But as the world war unfolded the B-24 program was moving along smartly. Early models were provided to the RAF; the U.S. Navy took an interest; and the USAAF began training and deployment of the aircraft. In addition to its primary role as a bomber, the B-24 was pressed into service in a variety of other capacities - notably as a transport, for ocean patrol, photo reconnaissance, and early versions of ferrets (used electronic eavesdropping). The key attributes of long range with adequate payload while flying at low- to medium altitudes fitted many of these vocations very well. And - as production ramped up - the B-24 had the invaluable attribute of being very widely available.But over time, limitations of the B-24 design began to emerge. These included inadequate high-altitude flying capability, structural weaknesses, and high control efforts for the pilots. As the B-24 design was evolved, these limitations became more pronounced due to the weight of added equipment, sail area of an elongated nose, and drag of added turrets. As happens to some military aircraft designs, over time an aircraft with very acceptable characteristics is overburdened such that its flying qualities decline. This was to be the fate of the B-24. But the war was an emergency; the B-24 was “good enough” to fight the war and the machinery of production and deployment had immense inertia. The B-24 would have to serve, and it did so with considerable distinction.In contrast early model B-17s turned in a mixed performance in combat. Twenty B-17C models sent to the RAF fared poorly for a number of reasons, not least of which was the lack of an RAF doctrine for using the aircraft. A group of unarmed B-17s literally flew into the Pearl Harbor attack and some were damaged and few on the ground destroyed. Others, based in the Philippines, fought a rag-tag war of attrition. In this role they earned an excellent reputation, particularly when the new tail-gun equipped E models entered combat. The Japanese very quickly learned that the B-17E was a formidable opponent and in fact called it a “four engine pursuit plane suitable for all purposes” - a very high accolade. However, all E- and early F-model B-17s lacked long-range “Tokyo tanks” in their outer wings, forcing the use of bomb bay tanks on long missions, cutting down the bombload. At the time B-24s did not have this issue, which led to the long-running myth that B-24s carried more bombs further than B-17s. In fact this was true for a time in the Pacific war and was a factor in deployment decisions. However, most of B-17 production had Tokyo tanks, which gave the aircraft essentially the same range-payload equation as the B-24.Nevertheless, during 1943 B-24s gradually replaced most B-17s in the Pacific. The primary reason was not range/payload, but the need for B-17s in the ETO. High Command determined that the more numerous B-24 should have priority for the Pacific theater, where ranges were long and altitudes flown relatively low - circumstances best suited to B-24 characteristics. Over Germany, where high altitude performance, toughness, defensive armament, and excellent formation flying capability were necessities, the B-17 was the natural choice. While B-17s were replaced in Pacific bomb groups, several remained in the theater as armed combat transports and executive transports. Beginning in 1945, as supply improved, B-17s were reintroduced into the theatre in substantial numbers. But from mid-1943 until large-scale introduction of the B-29, B-24s served as the heavy bomber in the theater and is recognized for its generally good performance against the Japanese.In the ETO things were different. The 8th Air Force was pioneered and led by B-17 groups. The few B-24 groups originally slated were quickly shifted to the Mediterranean Theater, with other B-24 groups joining later. Buildup of the 8th was slow initially as the logistics pipeline was established. Only 12 B-17Es conducted the first strategic bombing mission, to Rouen, France - a long, long stretch to 1000-plane raids on Berlin. The 8th had a lot to learn and a lot to do in taking on the formidable Nazi war machine. This job was primarily that of the B-17 alone. There were very few fighter escorts at first, those that were available had very short range, and the long, hard road to defeating Germany with daylight bombing had to be borne by B-17s. This would prove to be the toughest job in the history of aerial warfare and - now it appears clear - no other aircraft available at the time could have taken on this very costly duty.“She was a purple lady - but she packed a wallop” was the caption to a B-17G photo in one book on the air war. The reference to “purple” was, of course, to purple hearts: many were earned by B-17 crews. Too many were posthumous. B-17 build up continued from July 1942 until finally, in January 1943, raids on Germany proper began. Flying high in the stratosphere, leaving the escort behind at the fuel bingo point, tightening formation, guns at the ready, the B-17s droned on and on into the hostile sky. At some point the battle would be joined, and fast, heavily armed Nazi fighters - crewed by well-trained and intrepid pilots - challenged the intruding bombers. Not once was a B-17 formation turned back; but there were losses - on both sides. Over time, bomber losses became heavier. The reason, of course, is that they represented a dire threat to the Reich. This was the “schwerpunktbildung” - Prussian military language for “concentration of force at the point of attack” - and that is what both sides did. The result was the most desperate aerial battles in history.Into this maelstrom B-24s were introduced, forming a new division of the 8th AF. The Arsenal of Democracy was producing a great wealth of aircraft, and the plentiful B-24 was fighting on almost every front. As new groups were added to the 8th, they were given “milk run” missions to soft targets or flown as diversions while gaining experience. This period of working up was short. As soon as possible green crews and new combat units were sent into the mainstream of combat. Unfortunately B-17 and B-24 groups could not easily be operated together: the B-24 was slightly faster, particularly below 15,000 feet; but it could not fly above 23,000 feet with a full combat load. The B-17, on the other hand, easily flew at 25–30,000. Flying higher was safer from flak and put more pressure on Luftwaffe interceptors. One B-17 pilot later said, “you didn’t need to worry about flak and fighters when the B-24s were around.” But as time went on B-24s comprised about a third of the 8th’s bomber inventory. Like the B-17s above them, B-24s flew into the hell of combat.A vital aspect of the war was the logistics pipeline to the U.K. Protecting this pipeline was a critical necessity as the U-Boat fleet created havoc in 1942 and stood as an extreme threat to the Allied cause. Convoys needed close-in air cover and long-range aerial reconnaissance all across the Atlantic. Until B-24s were placed in this service, there was a gap in air coverage exploited by the Kriegsmarine. Again, the low-altitude range and endurance of the B-24 made it a natural choice for this duty, and both RAF Coastal Command and the USN used it for this purpose, flying out of a variety of bases - some quite inhospitable. The B-24 has been widely credited for its role in defeating the U-Boats, and served as arguably the most important single aircraft employed in the Battle of the Atlantic - although it certainly had a lot of help in the form of such aircraft as Catalinas, Sunderlands, and some Coastal Command B-17s.Nor was this the only battle facing the program. The second prototype aircraft was lost with famous engineering test pilot Eddie Allen, his crew, and about 20 victims on the ground when an inextinguishable engine fire spread to the wing in February 1943. The root cause was the underdeveloped Wright R-3350 engine - itself rushed into production - which at the time featured a magnesium alloy crankcase. A severe engine fire could ignite the magnesium, quickly becoming an uncontrollable inferno. Other problems dogged the program too - for example, organizing the huge supply base, tracking parts, and dealing with part quality and logistics. One accountant later said that just the parts he personally managed required a spreadsheet fifty feet long that was literally kept in a scroll. Such work required an army of clerks in an era of manual tabulation.The B-29 itself was a fantastic step function change from the B-17 and B-24. It was much larger and heavier, featured a Boeing-designed airfoil that incorporated laminar flow but avoided some of the weaknesses of the Davis wing; the crew cabin was pressurized; and bomber was equipped with a very sophisticated GE remote fire control system featuring analog computers. The wing itself was fitted with area-increasing fowler flaps, which helped enable the B-29 to carry roughly twice the bomb load twice as far at the prior heavy bombers while also being much faster. But it was a far more complex machine which demanded exacting crew coordination and required much longer and stronger runways, and larger hangars. Aviation infrastructure itself had to adapt to the new design.By mid-1943 the war in the Pacific had taken a firm turn in the Allies favor. The Battle of Guadalcanal had been won and the roll-back of the Japanese defense perimeter was underway. As the Allies advanced, bases could move forward. In this context B-24s based in New Guinea were flying missions to the Balikpapan oil installations so great in range that their bomb loads were cut to only 2500 pounds. The Pacific theater was vast - even the B-24 had trouble with the range requirements. But the mission profile was much less daunting than in the ETO - except for the long stretches over open ocean. This and many other dangers were ever present among heavy bomber crews.In Europe and the Mediterranean, however, the main air battles were just beginning. Two heavy bomber missions in August 1943 showed just how dangerous and costly these battles could be. On the first of August, B-24s from five groups based in Libya attacked Nazi-controlled oil infrastructure at Ploesti, Romania. This raid was brazenly flown at low level and some 53 of 177 aircraft were lost - thirty percent of the attacking force, taking with them 310 KIA and 190 captured or interned. No fewer than five medals of honor were conferred, the largest number ever for a single wartime action - three awarded posthumously. The refinery complex lost 40% capacity as a result of the raid, but output was restored in a few weeks. Clearly this outcome was far too low a return for such an investment in lives and equipment.The second costly August mission was the “anniversary raid” of the 17th, when 376 B-17s bombed Regensburg and Schweinfurt, Germany. The mission plan was complex: two large formations would attack the targets in coordination, with the Regensburg element flying on to North Africa instead of returning to the U.K. directly. It was hoped by this tactic that Nazi fighter opposition would be disrupted. As with all-too many operations conducted by the 8th, the weather proved to be another enemy; while the Regensburg element comprising 146 aircraft took off on time, the Schweinfurt element could not. Weather also disrupted routes and times for the Thunderbolt and Spitfire escort. Nevertheless, daring two-pronged mission - having already been scrubbed twice by weather - was continued despite the daunting odds.As the Regensburg raid developed, the bombers had very little escort due to missed rendezvous, weather, and fighter range limitations. Flying into heavy Luftwaffe opposition, the B-17s fought gallantly on to the target, which - at the time - was weakly defended by flak. The Messerschmidt aircraft plant target was decisively bombed; and the unexpected turn to the south gave the aircraft respite from further fighter attacks - but still 24 Fortresses were lost and many were damaged. Meantime the Schweinfurt element - already delayed by weather - ran into a cloud formation that forced the bombers down to 17,000 feet, far below the briefed altitudes of about 24–27,000 feet. Because of this factor, inadequate/missing escort, and new Luftwaffe tactics, 36 aircraft were lost. Bombing results were good, but - as with the oil facilities at Ploesti - production was badly disrupted instead of ended. Together the two B-17 attacking forces lost 60 aircraft - nearly sixteen percent of those dispatched.These two August raids collectively cost the USAAF 113 heavy bombers - an unacceptable overall loss rate of twenty percent. Further, in total 1,052 well-trained aircrew were killed, captured, or interned. This harsh reality illustrated just how dangerous Nazi defenses had become, as well as how disruptive weather could be. It also highlighted the need for more and better escorting fighters, as well as better fighter tactics and coordination with the bombers. Despite these realities, the war couldn’t wait. Potential solutions to the overarching problems would take time to develop and implement; meantime the vicious air war continued, and more adversity lay ahead.Life had taken a grim turn for U.S. heavy bomber crews, who lived in a Schizophrenic sort of paradigm where, after facing a high-probability of violent death during the day would return to their bases for a hot meal, a beer, and maybe even the company of a girlfriend. A close comrade in the next bunk may or may not be there the night after a bombing raid. The odds of making the required 25 missions to complete a tour were - at the time - distinctly unfavorable: in mid-1943 the average 8th AF crewman made only eleven missions before becoming a KIA, WIA or MIA statistic.In the world outside these tension-filled bomber bases, B-17s and B-24s went about many other duties: training, reconnaissance, transport, and experimental services. Generals were fond of having their own personalized B-17 for executive missions; B-24s were used as dedicated C-87 cargo and personnel transports; B-17s were prized for their bad-weather flying capability and were the weather reconnaissance aircraft of choice when available. Both aircraft were used for photomapping and ferreting. Crews of these aircraft had their own challenges to meet, although in general they were less demanding than those of direct combat. Nevertheless training crashes took their toll, as did bad weather. At the time, flying in general was both glamorous and dangerous. For 8th AF crewmen, the glamor had mostly become a cruel illusion, but they painted creative and often suggestive nose art on their airplanes anyway. If one is flying into the jaws of death, why not have a bare-chested maiden symbolizing your spirit? Aircraft with names such as Iza Vailable, Homesick Angel, Laden Maiden, and Snoozen Susin, took their crews into hell.Back in the USA, the XB-29 had made its maiden flight in September 1942 and continued in development as the overall program was given vast resources - more, in fact, than those given to the Manhattan Project. The B-29 program involved not only engineering and development, tooling, and new plants, but also a large network of new airfields designed to accommodate what would become the world’s most sophisticated aircraft - and would become the most costly U.S. wartime initiative. The scale of difficulty is very hard to adequately communicate. From a prototype in September 1942 to March 1944 the Program compressed what would normally take years into 18 months. The result was flyable but incomplete B-29s scattered over airfields near the Boeing Wichita plant waiting to be upgraded to combat-capable levels. Maintenance personnel, often working outside in inclement weather, undertook these modifications in the late winter-early spring season of 1944. This operation became known as the “Battle of Kansas.”While all this was going on, the B-29 program continued to mature. In April 1944 the 20th Air Force, based at Smokey Hill AAF near Salina, Kansas, began organizing for deployment. Across the far Midwest, with its open spaces, and generally good flying weather, large if remote bases for B-29s had sprung up. One such, Walker Army Airfield, is exemplary. Located on Kansas farmland in between the oil-patch town of Russell and the college town of Hays, this base was in the midst of rural America. There was little for off-duty crews to do; Walker was isolated among the nodding sunflowers bowing before the ever-present prairie winds flowing from the southwest.There B-17s - used for transition training - and newly built B-29s plied their trades. Three runways - so long and deep in concrete that the entire cement capacity of the Midwest was devoted to them during their construction - dominated the base. A large apron facing equally large hangars accommodated these aircraft. The infrastructure included paved roads, a water tower, administrative buildings, personnel quarters - and even a swimming pool. This sort of base was duplicated in equally rural and remote places like Dodge City, Garden City, Great Bend, and Independence, Kansas - among several others. The latent power of an America held down by the Great Depression had swung into action - and, from this remove - the scale of the B-29 program remains breathtaking.*** This answer is incomplete, it was inadvertently published. Please consider it to be a work in progress. Thanks. ***