Boeing XB-29 Superfortress

Last revised April 17, 2000

The Boeing B-29 Superfortress was the outstanding heavy bomber of the Second World War, and was one of the key instruments in the victory over Japan during the Pacific War. It was the first production aircraft to make extensive use of remotely-controlled defensive armament, the first production bomber to use a central fire control system, and was the first production bomber with fully-pressurized crew compartments. The B-29 is perhaps best remembered today as the aircraft which dropped the two atomic bombs which brought an end to the war in the Pacific.

The origin of the B-29 can be traced back to the Boeing Model 316 project, which had been derived directly from the XB-15 of 1934. It differed from the XB-15 primarily in the relocation of the wing from a low- to a high-mounted position on the fuselage and by the installation of a nosewheel undercarriage. Since the XB-15 had been seriously underpowered, the Model 316 was to have been powered by four 2000 hp Wright R-3350 Duplex Cyclone eighteen-cylinder air-cooled radials. These engines did not yet exist.

The next step was the Boeing Model 322 project of March 1938, which was a design study requested by the Army for a pressurized version of the B-17 with a tricycle landing gear. The Boeing company had prior experience with pressurized aircraft, having built the Model 307 Stratoliner commercial transport, the first pressurized airliner to enter commercial service. The Model 322 resembled the Stratoliner in some respects, combining a new, large-diameter fuselage with a standard B-17 wing and tail. The Model 322 featured a nosewheel undercarriage and was to be powered by four Pratt & Whitney R-2180 radials. A maximum speed of 307 mph at 25,000 feet was envisaged, and the maximum bomb load was to have been 9920 pounds. At that time, the Army lacked the funds to pursue the project officially, but Boeing was sufficiently inspired that the company decided to go forward with the project on its own.

Further work resulted in the Model 333A of late 1938. It was to have been powered by four 1150 hp Allison V-1710 twelve-cylinder liquid-cooled engines, installed in tandem pairs. However, full pressurization of the cabin was considered impractical because of the need to open the bomb bays during high-altitude flight, and it was decided that only the crew areas in the nose and in the mid-fuselage sections were to be pressurized. The nose and mid-fuselage pressurized cabins were to be connected by a pressurized tunnel passing over the bomb bay that allowed the crew members to change positions during pressurized flight. This feature remained on all subsequent Boeing long range bomber design proposals.

Because of the poor high-altitude performance of the Allison liquid-cooled engine, variations of the project were proposed with the new flat-mounted Wright and Pratt & Whitney radial engines. This led to the Model 333B project of February 1939. It was powered by four Wright engines buried in the thick wing. With a gross weight of 52,180 pounds, the maximum speed was to have been 364 mph at 20,000 feet. The range was to have been 2500 miles with a 2000-pound bombload.

In March of 1939, the Model 334 was proposed. The wingspan was extended to 120 feet in order to provide enough fuel to reach a range of 4500 miles. The Pratt & Whitney radials were still buried in the wing. A twin fin-and-rudder was to have been used to facilitate the installation of tail armament. Gross weight was up to 66,000 pounds and maximum bomb load was 7830 pounds.

In July of 1939, Boeing revised the design still further to produce the Model 334A. It eliminated the buried engine installation and the twin fin-and-rudder assembly of the Model 334 and replaced them with four conventionally-mounted Wright R-3350 radials and a single vertical tail. A high aspect ratio wing of 135 feet span was to be used. The Boeing proposal had finally begun to take the recognizable shape of what was eventually to emerge as the Superfortress.

With its own funds, Boeing built a mockup of its bomber design in December of 1939. It envisaged wing loadings as high as 64 pounds per square foot, a twelve-man crew and the ability to carry 2000 pounds of bombs over distances in excess of 5000 miles.

In the meantime, General Henry H. (Hap) Arnold, the acting head of the Army Air Corps, had become alarmed by the growing war clouds in Europe and by the Japanese military campaign in China. He established a special committee, chaired by Brigadier General W. G. Kilner, to make recommendations for the long term needs of the Army Air Corps. No less a personage than the famous aviator Charles Lindbergh had been a member of the committee. Lindbergh had recently toured German aircraft factories and Luftwaffe bases, and had become convinced that Germany was well ahead of its potential European adversaries. In their June 1939 report, the Kilner committee recommended that several new long-range medium and heavy bombers be developed. Hastened by a new urgency caused by the outbreak of war in Europe on September 1, on November 10, 1939, General Arnold requested authorization to contract with major aircraft companies for studies of a Very Long-Range (VLR) bomber that would be capable of carrying any future war well beyond American shores. Approval was granted on December 2, and USAAC engineering officers under Captain Donald L. Putt of the Air Material Command at Wright Field began to prepare the official specification.

In January of 1940, the Army issued the formal requirements for the VLR "superbomber". The requirements called for a speed of 400 mph, a range of 5333 miles, and a bomb load of 2000 pounds delivered at the halfway-point at that range. This became the basis for Request for Data R-40B and Specification XC-218. On January 29, 1940, the War Department formally issued Data R-40B and circulated it to Boeing, Consolidated, Douglas, and Lockheed. The official specification was revised in April to incorporate the lessons learned in early European wartime experience, and now included more defensive armament, armor, and self-sealing tanks.

The Boeing company had a leg up on its competition, since it had been thinking along the very same lines. In August of 1939, Boeing had started work on the Model 341 project, which featured a new high-lift aerofoil for a high aspect-ratio wing of 124 feet 7 inches in span. The Model 341 offered a maximum speed of 405 mph at 25,000 feet. It was to have been powered by four 2000 hp Pratt & Whitney R-2800 radials. Weighing 85,672 pounds, the range was to have been 7000 miles with one ton of bombs. A maximum load of 10,000 pounds could be carried over shorter distances.

In order to meet the requirements of Data R-40B, the Boeing Model 341 design was reworked into the Model 345. The Model 345 envisaged a pressurized aircraft, four Wright R-3350 engines replacing the R-2800s of the Model 341, a twelve-man crew, a double-wheeled tricycle undercarriage which retracted into the engine nacelles instead of sideways into the wing as on previous projects, four retractable turrets each carring a pair 0.5-inch machine guns, and a tail turret with two machine guns and a 20-mm cannon. The retractable Sperry power turrets were operated under remote control by gunners sighting through periscopes. The Model 345 was to be capable of carrying a ton of bombs over the stipulated 5333 miles at a cruising speed of 290 mph. The maximum bombload was to be 16,100 pounds. The maximum speed was estimated to be 382 mph at 25,000 feet. The weight was to be 97,700 pounds.

The Boeing Model 345 design was formally submitted to the Army on May 11, 1940. The Army was sufficiently interested that it awarded Boeing an appropriation for additional studies and wind-tunnel tests on June 17, and additional funds were obtained on June 27.

On June 27, 1940, the Army issued contracts for preliminary engineering data for the new "superbomber" to four manufacturers, which were designated in order of preference as Boeing XB-29, Lockheed XB-30, Douglas XB-31, and Consolidated XB-32. Seeing that they were at a competitive disadvantage, Lockheed and Douglas both subsequently withdrew from the competition before any detailed designs could be completed. On August 24, 1940, the Army ordered two prototypes and a static test model from Boeing under the designation XB-29. In addition, two XB-32 prototypes were ordered from Consolidated as insurance against the failure of the favored Boeing XB-29. The contract was dated September 6.

Full-scale mockups were ready for inspection by late November and the Army was so impressed by the Boeing submission that a third prototype was added to the contract on December 14.

Another mockup was started in May of 1941. With the growing likelihood of war coming to America, the B-29 project became more urgent, and 14 service-test examples were ordered under the designation YB-29. On May 17, 1941, the Army announced that an order would be placed for 250 B-29s to be built in a new government-owned facility at Wichita, Kansas that would be leased by Boeing for B-29 production. This order was confirmed in September of 1941. After Pearl Harbor and the entry of the USA into the war, the contract was increased to 500 in February of 1942. In March of 1942, over a thousand more B-29s were ordered and the Army specified that three other corporations would also participate in B-29 production-- the Bell Aircraft Company in a new plant in Marietta, Georgia, North American Aviation in a new plant at Kansas City, Missouri, and the Fisher Body Division of General Motors in its plant at Cleveland, Ohio. This order for a "paper" airplane that had not actually yet flown was equalled only by the order for the B-26 Marauder.

Before production actually began, it was decided that North American-Kansas City would concentrate on the B-25 Mitchell and that its place in the B-29 pool would be taken by another Boeing plant, this one located at Renton in the state of Washington. The Boeing-Renton plant had originally been built by the Navy for the manufacture of the Boeing PBB-1 Sea Ranger twin-engined patrol bomber seaplane. In a swap between the Navy and the USAAF, it was agreed that the PBB-1 project would be cancelled and that the Renton plant would be reallocated to B-29 production in exchange for the delivery of land-based bombers such as the B-25 Mitchell and the B-24 Liberator to the Navy.

It was later decided that Fisher should concentrate on the P-75 escort fighter plane, and its place in the B-29 production pool was taken by the Glenn L. Martin company, which was allocated a contract to build the B-29 in a new government-owned plant at Omaha, Nebraska.

In addition to prime airframe assembly at these four plants, a complex sub-contracting program was set up throughout the country for B-29 components and sub-assemblies. Companies such as Chrysler, Hudson, Goodyear, Briggs, Murray and Cessna were chosen to produce major B-29 airframe components.

The Model 345 design that eventually emerged featured a high-aspect ratio wing that was mid-mounted on a circular-section fuselage. The B-29 wing was tested in flight by substituting quarter-sized B-29 wing mockups for the regular wings of a Fairchild PT-19A trainer (serial number 41-20531). The wing loading of the B-29 was projected to be so high that special means would have to be taken to prevent the landing speed from being prohibitively high. This was done by using Fowler-type flaps to increase the lift coefficient of the wing. These flaps added 20 percent to the overall wing area when extended. The rear portions of the inner engine nacelles were extended aft of the wing trailing edges, which were modified to improve the flap characteristics. During design, the forward fuselage was extended, increasing overall length from 93 feet to 98 feet 2 inches, and the contours of the streamlined transparent nose were rounded off. A large dorsal forward extension was added to the vertical tail surfaces to improve asymmetric handling.

A remotely-controlled armament system had been adopted for the Model 345, since manned turrets were rejected as being impractical for the altitudes at which the B-29 would be operating. Four turrets were to be fitted, two on top and two underneath the fuselage, each with a pair of 0.5-inch machine guns. A fifth turret was in the tail and was under direct control of a tail gunner. It carried two 0.50-inch machine guns and one 20-mm cannon. Four companies competed for the contract to develop the armament system, including Bendix, General Electric, Sperry, and Westinghouse. The Sperry system involved the use of retractable turrets that were aimed by periscopic sights and won the initial contract.

Bombs were to be carried in two separate bomb bays, each with its own set of doors. The release of bombs was to be controlled through an intervalometer to preserve aircraft balance by alternating release between the bays.

The engine for the B-29 was the completely new 2200 hp Wright R-3350 Duplex Cyclone eighteen-cylinder twin row air-cooled radial. In order to gain the utmost power at high altitude, the engine was fitted with two turbosuperchargers instead of the usual one. The superchargers were General Electric B-11 units, automatically regulated by a Minneapolis-Honeywell electronic system. The engine was to drive a three-bladed 17-foot diameter propeller. The propeller efficiency was preserved by gearing the propeller to 35 revolutions for every 100 revolutions of the engine crankshaft. Special attention was paid to the nacelle designs to reduce aerodynamic drag. The oil coolers and the supercharger intercooler were mounted directly underneath the engine cooling air intake.

The construction of the B-29 was fairly conventional, being of all-metal throughout but with fabric-covered control surfaces. Each undercarriage unit had dual instead of single wheels. A retractable tail bumper was provided for tail protection during nose-high takeoffs and landings.

It was anticipated that the crew would vary from 10 to 14, but would normally consist of 12. The crew consisted of two pilots, a navigator, a bombardier, a flight engineer, a radio operator, a radar operator, and five gunners. The bombardier sat in the nose with his bombsight and gunsight. The pilot and copilot sat side-by-side behind panels of armor and bulletproof glass. The flight engineer, radio operator, and navigator sat immediately behind the pilot's cockpit. The rear pressurized compartment housed four gunners plus the radar operator, all protected by an armored bulkhead. The tail gunner sat in a separate pressurized compartment in the extreme rear of the aircraft. He could enter or leave the compartment only during unpressurized flight.

The first XB-29 (41-0002) flew on September 21, 1942 at Boeing Field, Boeing's chief test pilot Edmund T. "Eddie" Allen being at the controls. By this time, there were 1664 B-29 aircraft on order. No armament was initially fitted. The engines were four R-3350-12s with 17-foot diameter three-bladed propellers. Unfortunately, the early R-3350 engines were subject to chronic overheating and were specially prone to catching fire upon the slightest provocation. By December, Allen had been able to get only 27 hours in the air out of 23 test flights. Sixteen engines had to be changed, nineteen exhaust systems had to be revised, and twenty-two carburetors had to be replaced. There were also problems with the governor. On December 28, one of the R-3350 engines of the prototype caught fire during a test flight, forcing Allen to return immediately to Boeing Field. Aside from the engine problems, the performance and handling qualities of the B-29 were found to be excellent. Other than the rudder boost being removed, no significant aerodynamic changes were found to be necessary. The first XB-29 remained at Boeing throughout the war as a test aircraft.

The second XB-29 (41-0003) flew for the first time on December 30, 1942, but this flight was cut short by another engine fire, which caused a suspension of further tests until the engines could be replaced. The engines from XB-29 number 1 were removed and put in No. 2. The second XB-29 flew again on February 18, 1943, but an inextinguishable engine fire broke out just eight minutes into the flight, forcing an emergency return to the field. While attempting to land at Boeing Field, the fire burned through the main wing spars and caused the wing to buckle. The burning XB-29 plunged into the nearby Frye Meat Packing Plant factory, killing test pilot Eddie Allen and everyone else aboard, plus about 20 workers on the ground.

This crash caused ripples up the chain of command all the way to President Franklin Roosevelt, who was already unhappy about the delays in the B-29 program. He wanted B-29s on their way to India by the end of 1943 so that they could begin bombing attacks against Japan. Senator Harry Truman's Special Committee to Investigate the National Defense Program, which had been established to expose fraudulent overcharging and other violations in defense acquisitions, looked into the B-29 program and concluded that the problem lay with substandard or defective engines delivered by the Wright Aeronautical Company. The USAAF also came in for a share of the blame, by having put too much pressure on the Wright company to speed up engine delivery.

The third prototype (41-18335) flew for the first time in June of 1943. It incorporated extensive powerplant and equipment revision as a result of experience with the first two. It was sent to Wichita to assist in the establishment of the production line. It was soon handed over to the USAAF for armament and accelerated flight testing. It too eventually crashed, but not before verifying the potential of basic design.

Serials of XB-29

41-0002/0003		Boeing XB-29 Superfortress
41-18335		Boeing XB-29 Superfortress


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