Boeing B-29 Superfortress - Design and Development

Design and Development

Boeing began work on pressurized long-range bombers in 1938, when, in response to a United States Army Air Corps request, it produced a design study for the Model 334, a pressurized derivative of the Boeing B-17 Flying Fortress with nosewheel undercarriage. Although the Air Corps did not have money to pursue the design, Boeing continued development with its own funds as a private venture, so that when, in December 1939, the Air Corps issued a formal specification for a so-called "superbomber", capable of delivering 20,000 lb (9,100 kg) of bombs to a target 2,667 mi (4,290 km) away and capable of flying at a speed of 400 mph (640 km/h), they formed a starting point for Boeing's response.

Boeing submitted its Model 345 on 11 May 1940, in competition with designs from Consolidated Aircraft (the Model 33, later to become the B-32), Lockheed (the Lockheed XB-30), and Douglas (the Douglas XB-31). Douglas and Lockheed soon abandoned work on their projects, but Boeing received an order for two flying prototypes, 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 in case of problems with Boeing's design. An initial production order for 14 service test aircraft and 250 production bombers was placed in May 1941, this being increased to 500 aircraft in January 1942. 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 having the only "stepless" cockpit design, without a separate windscreen for the pilot, on an American combat aircraft of World War II.

Manufacturing the B-29 was a complex task. It involved four main-assembly factories: a pair of Boeing operated plants at Renton, Washington, and Wichita, Kansas, a Bell plant at Marietta, Georgia ("Bell-Atlanta"), and a Martin plant at Omaha, Nebraska ("Martin-Omaha"). Thousands of subcontractors were involved in the project. The first prototype made its maiden flight from Boeing Field, Seattle on 21 September 1942. Because of the aircraft's highly advanced design, challenging requirements, and immense pressure for production, development was deeply troubled. The second prototype, which, unlike the unarmed first, was fitted with a Sperry defensive armament system using remote-controlled gun turrets sighted by periscopes, first flew on 30 December 1942, this flight being terminated due to a serious engine fire. On 18 February 1943, the second prototype experienced an engine fire and crashed. 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. The Air Force–operated modification depots struggled to cope with the scale of work required, with a lack of hangars capable of housing the B-29 combined with freezing cold weather further delaying the modification, such that at the end of 1943, although almost 100 aircraft had been delivered, only 15 percent were airworthy. 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 modification 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 six weeks between 10 March and 15 April 1944.

The most common cause of maintenance headaches and catastrophic failures was the engine. 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 installed around rubber push rod fittings to prevent oil loss, thorough pre-flight inspections made to detect unseated valves, and frequent replacement of the uppermost five cylinders (every 25 hours of engine time) and the entire engines (every 75 hours).

Pilots, including the present day pilots of the Commemorative Air Force’s Fifi, the last remaining flying B-29, 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 rolling rather than from a "braked" start.

In wartime, the B-29 was capable of flight up to 31,850 feet (9,710 m), at speeds of up to 350 mph (560 km/h) (true airspeed). This was its best defense, because Japanese fighters of that day could barely get that high, and few could catch the B-29, even if they were at altitude and waiting. Only the heaviest of anti-aircraft weapons could reach it, and since the Axis forces did not have proximity fuzes, hitting or damaging the aircraft from the ground in combat was next to impossible.

The B-29's revolutionary Central Fire Control system included four remotely controlled turrets armed with two .50 Browning M2 machine guns each. All weapons were aimed electronically from five sighting stations located in the nose and tail positions and three Perspex 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. The gunner in the upper position acted as fire control officer, managing the distribution of turrets among the other gunners during combat.

In early 1945, with a change of role from high-altitude day bomber to low-altitude night bomber, LeMay reportedly ordered the removal of most of the defensive armament and remote-controlled sighting equipment from his B-29s so that they could carry greater fuel and bomb loads.As a consequence of this requirement, Bell Marietta (BM) produced a series of 311 B-29Bs that had turrets and sighting equipment removed, except for the tail position, which initially had the two .50 cal Browning machine guns and single M2 cannon with the APG-15 radar fitted as standard. This armament was quickly changed to three .50 caliber Brownings. This version also had an improved APQ-7 "Eagle" bombing-through-overcast radar fitted in an airfoil shaped radome under the fuselage. Most of these aircraft were assigned to the 315th Bomb Wing, Northwest Field, Guam.

The crew enjoyed, for the first time in a bomber, full-pressurization comfort. This first-ever cabin pressure system for an Allied production bomber was developed for the B-29 by Garrett AiResearch. The nose and the cockpit were pressurized, but the designers were faced with deciding whether to have bomb bays that were not pressurized, between fore and aft pressurized sections, or a fully pressurized fuselage with the need to de-pressurize to drop their loads. The decision was taken to have a long tunnel over the two bomb bays so that crews could crawl back and forth between the fore and aft sections, with both areas and the tunnel pressurized. The bomb bays were not pressurized.

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

• The view of the cockpit from the rear, illustrating the visibility achieved by the array of windows

• The flight engineer's panel is where the engines and systems are controlled.

• The navigator's station, set up to portray a mission over Japan.

• A B-29 crew illustrating the jobs it has to perform.