The Boeing B-29 Superfortress is a retired American four-engined propeller-driven heavy bomber, designed by Boeing and flown primarily by the USAAF and USAF during World War II and the Korean War. Named in allusion to its predecessor, the Boeing B-17 Flying Fortress, the Superfortress was designed for high-altitude strategic bombing, but also excelled in low-altitude night incendiary bombing and in dropping naval mines to blockade Japan. Silverplate B-29s dropped the atomic bombs on Hiroshima and Nagasaki, and remain the only system to deliver nuclear weapons in combat.
One of the largest aircraft of World War II, the B-29 was designed with state-of-the-art technology, which included a pressurized cabin, dual-wheeled tricycle landing gear, and an analog computer-controlled fire-control system that allowed one gunner and a fire-control officer to direct four remote machine gun turrets. The $3 billion cost of design and production (equivalent to $ billion in ), far exceeding the $1.9 billion cost of the Manhattan Project, made the B-29 program the most expensive of the war. The B-29 remained in service in various roles throughout the 1950s, being retired in the early 1960s after 3,970 had been built. A few were also used as flying television transmitters by the Stratovision company. The Royal Air Force flew the B-29 with the service name Washington from 1950 to 1954, when the jet-powered Canberra entered service.
The B-29 was the progenitor of a series of Boeing-built bombers, transports, tankers, reconnaissance aircraft, and trainers. For example, the re-engined B-50 Superfortress Lucky Lady II became the first aircraft to fly around the world non-stop, during a 94-hour flight in 1949. The Boeing C-97 Stratofreighter airlifter, which was first flown in 1944, was followed in 1947 by its commercial airliner variant, the Boeing Model 377 Stratocruiser. In 1948, Boeing introduced the KB-29 tanker, followed in 1950 by the Model 377 derivative KC-97. A line of outsized-cargo variants of the Stratocruiser is the GuppyMini GuppySuper Guppy, which remain in service with NASA and other operators. The Soviet Union produced 847 Tupolev Tu-4s, an unlicensed reverse-engineered copy of the B-29. Twenty-two B-29s have survived to preservation. The majority are on static display at museums, while two airframesFIFI and Docstill fly.
Design and development
Boeing began work on long-range bombers in 1938. Boeing's design study for the Model 334 was a pressurized derivative of the Boeing B-17 Flying Fortress with nosewheel undercarriage. Although the Air Corps lacked funds to pursue the design, Boeing continued development with its own funds as a private venture.Bowers 1989, p. 318. In December 1939, the Air Corps issued a formal specification for a so-called "superbomber" that could deliver of bombs to a target away, and at a speed of . Boeing's previous private venture studies formed the starting point for its response to the Air Corps formal specification.Willis 2007, pp. 136–137.
On 29 January 1940, the United States Army Air Corps issued a request to five major aircraft manufacturers to submit designs for a four-engine bomber with a range of . Boeing submitted its Model 345 on 11 May 1940,Bowers 1989, p. 319. in competition with designs from Consolidated Aircraft (the Model 33, which later became the B-32),Wegg 1990, p. 91. Lockheed (the Lockheed XB-30),"Factsheet: Lockheed XB-30." National Museum of the United States Air Force. Retrieved: 15 November 2010. and Douglas (the Douglas XB-31).Francillon 1979, p. 713.
Douglas and Lockheed soon abandoned work on their projects, but Boeing received an order on 24 August 1940 for two flying prototypes, which were given the designation XB-29, and an airframe for static testing. The order was revised to add a third flying aircraft on 14 December. Consolidated continued to work on its Model 33, as it was seen by the Air Corps as a backup if there were problems with Boeing's design.Willis 2007, p. 138. These designs were evaluated, and on 6 September orders were placed for two experimental models each from Boeing and Consolidated, which became the Boeing B-29 Superfortress and the Consolidated B-32 Dominator. These were known as very long range (VLR) bombers; the name "Superfortress" was not assigned until March 1944. On 17 May 1941, Boeing received an initial production order for 14 service test aircraft and 250 production bombers;Knaack 1988, p. 480. this being increased to 500 aircraft in January 1942.
Manufacturing the B-29 was a complex task that involved four main-assembly factories. There were two Boeing operated plants at Renton, Washington (Boeing Renton Factory), and one in Wichita, Kansas, a Bell plant at Marietta, Georgia, near Atlanta ("Bell-Atlanta"), and a Martin plant at Bellevue, Nebraska ("Martin-Omaha" – Offutt Field).Bowers 1989, p. 322. Thousands of subcontractors were also involved in the project.Willis 2007, pp. 138–139. The first prototype made its maiden flight from Boeing Field, Seattle, on 21 September 1942. The combined effects of the aircraft's highly advanced design, challenging requirements, immense pressure for production, and hurried development caused setbacks. Unlike the unarmed first prototype,Brown 1977, p. 80. the second was fitted with a Sperry defensive armament system using remote-controlled gun turrets sighted by periscopes. It first flew on 30 December 1942, although the flight was terminated due to a serious engine fire.Peacock Air International August 1989, pp. 70–71.
On 18 February 1943, the second prototype, flying out of Boeing Field in Seattle, experienced an engine fire and crashed. The crash killed Boeing test pilot Edmund T. Allen and his 10-man crew, 20 workers at the Frye Meat Packing Plant, and a Seattle firefighter. Changes to the production craft came so often and so fast that, in early 1944, B-29s flew from the production lines directly to modification depots for extensive rebuilds to incorporate the latest changes. AAF-contracted modification centers and its own air depot system struggled to handle the scope of the requirements. Some facilities lacked hangars capable of housing the giant B-29, requiring outdoor work in freezing weather, further delaying necessary modification. By the end of 1943, although almost 100 aircraft had been delivered, only 15 were airworthy.Willis 2007, p. 144.Peacock Air International August 1989, p. 76. This prompted an intervention by General Hap Arnold to resolve the problem, with production personnel being sent from the factories to the modification centers to speed availability of sufficient aircraft to equip the first bomb groups in what became known as the "Battle of Kansas". This resulted in 150 aircraft being modified in five weeks, between 10 March and 15 April 1944.Knaack 1988, p. 484.Bowers 1989, p. 323.Herman 2012, pp. 284–346.The most common cause of maintenance headaches and catastrophic failures was the engines. Although the Wright R-3350 Duplex-Cyclone radial engines later became a trustworthy workhorse in large piston-engined aircraft, early models were beset with dangerous reliability problems. This problem was not fully cured until the aircraft was fitted with the more powerful Pratt & Whitney R-4360 "Wasp Major" in the B-29D/B-50 program, which arrived too late for World War II. Interim measures included cuffs placed on propeller blades to divert a greater flow of cooling air into the intakes, which had baffles installed to direct a stream of air onto the exhaust valves. Oil flow to the valves was also increased, asbestos baffles were installed around rubber push rod fittings to prevent oil loss, thorough pre-flight inspections were made to detect unseated valves, and mechanics frequently replaced the uppermost five cylinders (every 25 hours of engine time) and the entire engines (every 75 hours).
During development, the high-altitude performance of the B-29's Wright R-3350 engines was severely limited by turbine blade failures inside their dual General Electric turbosuperchargers, where exhaust gases exceeded 1,500 °F (815 °C). To survive these extreme temperatures without warping or undergoing thermal creep, GE aviation pioneer Dr. Sanford Moss introduced Vitallium (later designated Haynes Stellite 21), a high-strength cobalt-chromium-molybdenum superalloy. Originally developed in 1932 by Austenal Laboratories as a lightweight, biocompatible, and corrosion-resistant material for dental dentures and orthopedic implants, Vitallium possessed the precise high-temperature metallurgical stability required for aviation. Utilizing the same "lost-wax" precision investment casting technique employed by the dental industry, GE was able to mass-produce the alloy into highly durable turbine "bucket wheels." This cross-industry metallurgical innovation provided the B-29 with reliable high-altitude turbocharging capabilities, giving the United States a critical strategic advantage over Axis powers, who lacked access to the strategic cobalt and chromium reserves needed to manufacture comparable high-temperature alloys.
Pilots, including the present-day pilots of the Commemorative Air Force's Fifi, one of the last two remaining flying B-29s, describe flight after takeoff as being an urgent struggle for airspeed (generally, flight after takeoff should consist of striving for altitude). Radial engines need airflow to keep them cool, and failure to get up to speed as soon as possible could result in an engine failure and risk of fire. One useful technique was to check the magnetos while already on takeoff roll rather than during a conventional static engine-runup before takeoff.Gardner, Fred Carl "A Year in the B-29 Superfortress." Fred Carl Gardner's website, updated 1 May 2005. Retrieved: 11 April 2009.
The $3 billion cost of design and production (equivalent to $ billion in 2022), far exceeding the $1.9 billion cost of the Manhattan Project, made the B-29 program the most expensive of the war. The unit cost was $639,188 ($ million in ),Knaack 1988, p. 486. and the prototype cost was $3,392,396.60 ($ million in ).
Features Defensive gun turret emplacements
In wartime, the B-29 was capable of flight at altitudes up to ,"B-29 Superfortress." Boeing. Retrieved: 22 March 2012. at speeds of up to (true airspeed). This was its best defense because Japanese fighters could barely reach that altitude, and few could catch the B-29 even if they did attain that altitude.
The General Electric Central Fire Control system on the B-29 directed four remotely controlled turrets armed with two .50 Browning M2 machine guns each. All weapons were aimed optically, with targeting computed by analog electrical instrumentation. There were five interconnected sighting stations located in the nose and tail positions and three Plexiglas blisters in the central fuselage. Five General Electric analog computers (one dedicated to each sight) increased the weapons' accuracy by compensating for factors such as airspeed, lead, gravity, temperature and humidity. The computers also allowed a single gunner to operate two or more turrets (including tail guns) simultaneously.