Boeing B-29 Superfortress

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The Boeing B-29 Superfortress is an American four-engined propeller-driven heavy bomber, designed by Boeing and flown primarily by the United States during World War II and the Korean War. Named in allusion to its predecessor, the 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. B-29s dropped the atomic bombs on Hiroshima and Nagasaki, the only aircraft ever to drop 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 $45 billion today),[3] far exceeding the $1.9 billion cost of the Manhattan Project, made the B-29 program the most expensive of the war.[4][5] 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 as the Washington until 1954.

The B29 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. This bomber-to-airliner derivation was similar to the B-17/Model 307 evolution. 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 Guppy / Mini Guppy / Super 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 B-29s remain as static displays, but only two, FIFI and Doc, still fly.[6]

Design and development
Before World War II, the United States Army Air Corps concluded that the Boeing B-17 Flying Fortress, which would be the Americans' primary strategic bomber during the war, would be inadequate for the Pacific Theater, which required a bomber that could carry a larger payload more than 3,000 miles.[7]


The length of the 141-foot (43 m) wing span of a Boeing B-29 Superfortress based at Davis-Monthan Field is vividly illustrated here with the cloud-topped Santa Catalina Mountains as a contrasting background.
Two large olive-colored aircraft flying over farmland
YB-29 Superfortresses in flight

1000th B-29 delivery ceremony at Boeing Wichita plant in February 1945
In response, Boeing began work on pressurized 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.[8] In April 1939, Charles Lindbergh convinced General Henry H. Arnold to produce a new bomber in large numbers to counter the Germans' bomber production.[9] In December 1939, the Air Corps issued a formal specification for a so-called "superbomber" that could deliver 20,000 lb (9,100 kg) of bombs to a target 2,667 mi (4,292 km) away, and at a speed of 400 mph (640 km/h). Boeing's previous private venture studies formed the starting point for its response to the Air Corps formal specification.[10]

Boeing submitted its Model 345 on 11 May 1940,[11] in competition with designs from Consolidated Aircraft (the Model 33, which later became the B-32),[12] Lockheed (the Lockheed XB-30),[13] and Douglas (the Douglas XB-31).[14] Douglas and Lockheed soon abandoned work on their projects, but Boeing received an order for two flying prototypes, which were given the designation XB-29, and an airframe for static testing on 24 August 1940, with the order being 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.[15] Boeing received an initial production order for 14 service test aircraft and 250 production bombers in May 1941,[16] this being increased to 500 aircraft in January 1942.[11] The B-29 featured a fuselage design with circular cross-section for strength. The need for pressurization in the cockpit area also led to the B-29 being one of very few American combat aircraft of World War II to have a stepless cockpit design, without a separate windscreen for the pilots.

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 (now Spirit AeroSystems), a Bell plant at Marietta, Georgia, near Atlanta ("Bell-Atlanta"), and a Martin plant at Omaha, Nebraska ("Martin-Omaha" – Offutt Field).[11][17] Thousands of subcontractors were also involved in the project.[18] The first prototype made its maiden flight from Boeing Field, Seattle, on 21 September 1942.[17] 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,[19] the second was fitted with a Sperry defensive armament system using remote-controlled gun turrets sighted by periscopes and first flew on 30 December 1942, although the flight was terminated due to a serious engine fire.[20]

On 18 February 1943, the second prototype, flying out of Boeing Field in Seattle, experienced an engine fire and crashed.[20] 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.[21] 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.[22][23] 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 the five weeks, between 10 March and 15 April 1944.[24][25][26]

The most common cause of maintenance headaches and catastrophic failures was the engines.[24] 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).[N 1][24][27]

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.[27]


Interior photo of the rear pressurized cabin of the B-29 Superfortress, June 1944

B-29 Weapons Bay with General-Purpose AN-M64 TNT 500 LB bombs
In wartime, the B-29 was capable of flight at altitudes up to 31,850 feet (9,710 m),[28] at speeds of up to 350 mph (560 km/h; 300 kn) (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.

Defensive gun turret emplacements

Tail armament, B-29 Superfortress, Hill Aerospace Museum

Gunner sighting station blister
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.[N 2] 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.[N 3] 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. The gunner in the upper position acted as fire control officer, managing the distribution of turrets among the other gunners during combat.[29][30][31][32] The tail position initially had two .50 Browning machine guns and a single M2 20 mm cannon. Later aircraft had the 20 mm cannon removed,[33] sometimes replaced by a third machine gun.[34]

In early 1945, Major General Curtis Lemay, commander of XXI Bomber Command—the Marianas-based B-29-equipped bombing force—ordered most of the defensive armament and remote-controlled sighting equipment removed from the B-29s under his command. The affected aircraft had the same reduced defensive firepower as the nuclear weapons-delivery intended Silverplate B-29 airframes and could carry greater fuel and bomb loads as a result of the change. The lighter defensive armament was made possible by a change in mission from high-altitude, daylight bombing with high explosive bombs to low-altitude night raids using incendiary bombs.[35] As a consequence of that requirement, Bell Atlanta (BA) produced a series of 311 B-29Bs that had turrets and sighting equipment omitted, except for the tail position, which was fitted with AN/APG-15 fire-control radar.[36] That version could also have an improved APQ-7 "Eagle" bombing-through-overcast radar fitted in an airfoil-shaped radome under the fuselage. Most of those aircraft were assigned to the 315th Bomb Wing, Northwest Field, Guam.[37]