Boeing B-29 Superfortress XB-29

The XB-29 story begins before any official requirement existed. Boeing had been studying long-range bombers since 1938, and its Model 334 was a pressurised derivative of the B-17 Flying Fortress with a nosewheel undercarriage. The Air Corps had no money to back it, so the company pressed on at its own expense as a private venture. That early investment paid off: when the Air Corps issued a formal specification in December 1939 for a "superbomber" able to carry 20,000 lb of bombs to a target 2,667 miles away at 400 mph, Boeing already had a foundation for its answer.

On 29 January 1940 the United States Army Air Corps asked five major manufacturers for designs of a four-engined bomber with a range of 2,000 miles. Boeing submitted its Model 345 on 11 May 1940, against Consolidated's Model 33 (the seed of the B-32), the Lockheed XB-30 and the Douglas XB-31. Lockheed and Douglas soon dropped out; Consolidated stayed in as the Air Corps' insurance in case Boeing's design ran into trouble. On 24 August 1940 Boeing received an order for two flying prototypes, designated XB-29, plus an airframe for static testing, and on 14 December a third flying aircraft was added. The name "Superfortress" would not be assigned until March 1944.

The design accepted as the XB-29 already incorporated improvements over the original submission, including more and larger guns and self-sealing fuel tanks. A full-scale mockup was completed in the spring of 1941. On 17 May 1941, with the prototype still under construction, Boeing received an initial order for 14 service-test aircraft and 250 production bombers, increased to 500 aircraft in January 1942. The bomber, in other words, was bought in quantity before it had flown.

The XB-29 combined several novelties at once: a new wing, a new type of remotely controlled defensive armament, pressurised crew stations and the powerful new Wright R-3350 radials driving slow-turning Hamilton Standard propellers of 16 ft diameter. Going from inception to mass production took three years, at a time when a project of that ambition would normally have needed five years merely to produce a prototype. Engineering, production and testing ran simultaneously, which multiplied both the expected and the unexpected problems.

Pressurisation deserves a note of its own. It was the first cabin-pressure system for an Allied production bomber, developed for the aircraft by Garrett AiResearch. The designers had to choose between a fully pressurised fuselage, which would have to be depressurised before the bomb-bay doors opened, and unpressurised bomb bays. They chose the latter and linked the forward and rear crew compartments with a long tunnel through which crew members could crawl.

The first XB-29, serial 41-1002, made its maiden flight from Boeing Field in Seattle on 21 September 1942. It flew unarmed. The second prototype carried a Sperry defensive system with remotely controlled turrets sighted through periscopes, a different solution from the one that would eventually be adopted for production. It first flew on 30 December 1942, on a flight cut short by a serious engine fire, a warning of what was to come. The XB-29s used R-3350-13 engines and three-bladed propellers, two features that would distinguish them from the later YB-29s.

On 18 February 1943 the second prototype, 41-1003, took off from Boeing Field for a two-hour powerplant performance test with Edmund T. "Eddie" Allen, Boeing's chief test pilot and head of its Research Division, at the controls. Fuel leaking from a filler cap in the wing leading edge ran down inside the leading edge and ignited; the fire spread to the engines and the aircraft, with much reduced power and unable to climb, crashed into the Frye meat-packing plant. All eleven crew members died, along with some twenty workers at the plant and a Seattle firefighter. The loss of Allen and other key Boeing engineers was a blow to the entire programme.

General Hap Arnold ordered an investigation, which found numerous problems with the basic engine design; the R-3350's overheating was never fully cured and stayed with the B-29 for the rest of the war. The Army Air Forces and a congressional committee chaired by then-Senator Harry S. Truman examined the programme and issued a scathing report that prompted the Army Air Forces to take control. Soon after the crash Colonel Harman proposed a plan to coordinate the process of working the bugs out of the aircraft, above all the engines, bringing production, modification, flight test and training under one authority; approval for the "B-29 Special Project" came directly from President Roosevelt on Arnold's advice.

The programme's cost shows how much was riding on the prototype: design and production came to $3 billion, more than the $1.9 billion spent on the Manhattan Project, making the B-29 the most expensive programme of the war. The prototype alone cost $3,392,396.60.

Changes to the production aircraft came so thick and fast that in early 1944 B-29s were flying straight from the assembly lines to modification depots. By the end of 1943 almost 100 aircraft had been delivered, but only 15 were airworthy. Arnold's intervention produced the "Battle of Kansas": factory personnel sent to the modification centres got 150 aircraft ready in five weeks, between 10 March and 15 April 1944, in time to equip the first bomb groups. The B-29 went into combat in June 1944 with its engines still not fully tamed.

The XB-29 was the starting point for the whole Superfortress family, and it was the first to test the pressurisation, the highly loaded wing and the R-3350 engines that defined the bomber. Its flight manual, the XB-29 Handbook, survives in the digital collections of the Museum of Flight in Seattle, the city where it first flew and where the second prototype was lost.