One of the striking facets of the Second World War was that the US deployed two generations of heavy strategic bombers during the conflict. Excepting the British, none of the other combatants managed this feat. All of the combatants had some plans for heavy four-engined bombers, but the US was able to make the transition between designing these aircraft and fielding them on a truly massive scale. The progressive advance of the US bomber arm was not necessarily a given. Strategic bombers were an expensive investment and the cost to establish B-29 production rivaled that of the Manhattan Project. The fact that the Army Air Corps (AAC) was able to accomplish this was the result of several interlocking factors in the interwar period.

A major factor working in favor of the heavy bomber was that the AAC's leadership was very pro-bomber. This was not uncommon for the interwar period among various air arms as this was that era of the so-called "bomber barons" like the RAF's Hugh Trenchard who believed that bombers could be a decisive weapon of the future. The AAC's Billy Mitchell was just one of several bomber-orientated officers who managed to set their mark on the AAC's doctrine and procurement. While there were some fighter-orientated officers like Claire Chennault, the AAC did spend a good deal of its effort on the idea of strategic bombing and bombers.

Where the AAC departed ways slightly with its contemporaries was that it conceptualized strategic bombing less as a tool of overwhelming force but rather as a precision instrument that could apply bombs at a critical faultline in an enemy force. This scientific approach to aerial warfare in part reflected the larger importance of Taylorist-style thinking within American intellectual circles. Taylorism stressed compartmentalization and specialization to create efficiencies, thus when the interwar planners conceptualized destroying an enemy's industrial capacity they looked to such bottlenecks. But the focus on precision also underscored the precariousness of AAC funding. AAC leaders needed to build up a strategy and doctrine around the budgets they could expect from an interwar Congress. The AAC already had a strong and entrenched rival in the form of the US Navy. Not surprisingly, the AAC's chiefs would argue that their force of land-based bombers could render the expensive battleships obsolete. Mitchell's publicized "tests" on obsolete battleship (which he rigged in a number of ways) were one example of how the AAC sought to make itself relevant against its naval rival.

The need to project power out into the ocean as well as carry a significant tonnage of bombs to the right place favored the development of large, four-engined bombers. The AAC not only positioned its bomber force as a war-winning offensive weapon but also as a proactive defensive weapon that could defend America's shores from a hostile naval force. The twin-engined bombers of the 1920s though could not fulfill either mission, so the AAC naturally turned to ever-larger aircraft. The winner of the 1934 design competition, Army Circular Proposal 35-356, for a new strategic bomber was the Boeing company's Model 299, later rechristened YB-17.

Boeing was something of an upstart company in the mid-1930s and the resulting B-17 emerged out of a series of false starts. It was already producing a heavy bomber for the AAC, the XB-15 as part of an earlier quest by the AAC to create a massive strategic bomber. The XB-15 was something of a dud but it gave the Boeing company a degree of confidence in designing large aircraft. The failures of US aviation companies to field a suitable heavy bomber led 35-356's requirements to be more realistic. Nonetheless the failures of the preceding heavy bomber projects did engender a degree of skepticism about the whole idea. 35-356 did not specify four-engines and Boeing's main rival for the project Douglas tendered a twin-engined design the DB-1. Boeing's management though proved willing to risk the company's fate on Model 299.

Part of the reason that Boeing was more willing to take these type of risks was that these large four-engined bombers dovetailed with what its board saw as the future of the civilian market. Bombers with high service ceilings, heavy payloads, and great operational range meant the development of technologies and designs for airliners that could service beyond regional routes, fly above the weather, and carry enough passengers to make such an investment economical. Boeing was able to parlay some of its experience into the YB-17 into its Model 307 Stratoliner which was a four-engined, pressurized civilian airliner. Now Boeing was not the only company thinking along these lines; both Lockheed and Douglas were pursuing their own large airliner projects. But Boeing was clearly intertwining its current military designs with potential civil ones and this made the company somewhat less risk-averse to pushing the envelope to meet military requirements despite the headaches such contracts entailed.

The developmental line between the B-17 and B-29 though was not a straight one though. The YB-17 had a number of teething problems and crashes and the AAC often failed in its tests at long-range reconnaissance. The AAC's reputation was also blackened by its failures to meet its promises to deliver air mail. Had such failures occurred in the prior decade the AAC's bomber program might have been in serious jeopardy from budget cuts. But the use of airpower in by the Japanese in China as well as by all sides in the Spanish Civil War underscored the importance of maintaining an air arm. The AAC soon found it had a firm ally within the Roosevelt administration as FDR was a keen proponent of building a strong air arm. This period also witnessed the promotion of a second generation of AAC officers like Hap Arnold, James Doolittle, and Carl Spaatz who had cut their teeth in the battles to establish the service in the 1920s and had a working relationship with industry.

So the AAC's leadership was primed to look for the next generation of strategic bombers even as its current generation of B-17s and C-212 (later known as the B-24) were just coming into their own. War clouds over Europe and the Pacific gave impetus for the AAC to form the Kilner Committee in January 1939 to plan out long-range needs of the AAC. The Kilner report in June 1939 recommended several proposals, foremost among them was the idea of a VLR (Very Long Range) bomber whose performance exceeded that of the B-17/24s entering service. Boeing was already thinking along these lines of the VLR and its experience with the Model 307 led to its production of a paper proposal, the Model 334 as well as other potential designs such as the Model 341/345. Boeing was thus able to get a leg up on its Douglas and Lockheed competition by having a proposal already in hand that matched the AAC's requirements. Only Consolidated went through the VLR competition as Lockheed and Douglas dropped out before producing a prototype. Consolidated was fighting a losing battle against Boeing, but its VLR was a development of its B-24 aircraft.

Boeing's victory in the VLR competition was not preordained though. The resulting B-29 was an expensive plane plagued by teething problems that bedeviled it through 1945 and beyond. But Boeing and its product had an advantage in that it was producing the type of plane that the AAC envisioned as ideal for this new form of warfare. Boeing's management and designers also interweaved its military designs with its civilian ones which made the company much more proactive when it came to AAC calls for bomber proposals. The AAC also developed a coherent vision of strategic airpower that not only called for an investment in large and expensive planes, but also justified the AAC's importance within the Congressional pecking order for the military budget. The larger geopolitical context of the mid-1930s in which aggressive nations like Germany and Japan used aircraft to deadly effect allowed the AAC to overcome most of its funding hurdles despite the ongoing Depression. It was much easier to "sell" the idea of a large bomber force to both FDR and Congress in light of the Condor Legion's use of bombing or the potential war in the Pacific. The large technical and scientific infrastructure of the US coupled with an assertive aviation sector of the economy meant that companies like Boeing could resolve most of the technical problems associated with pushing the envelope of aviation. This process though was expensive, but it was feasible.

Sources

Biddle, Tami Davis. Rhetoric and Reality in Air Warfare: The Evolution of British and American Ideas About Strategic Bombing, 1914-1945. Princeton University Press, 2002.

Kelsey, Benjamin S. The Dragon's Teeth?: The Creation of United States Air Power for World War II. Washington, D.C.: Smithsonian Institution Press, 1982.

Wolk, Herman S. Cataclysm: General Hap Arnold and the Defeat of Japan. Denton: Univ Of North Texas Press, 2012.