The Boeing XB-39 Superfortress was a

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

prototype

bomber A bomber is a military combat aircraft designed to attack ground and naval targets by dropping air-to-ground weaponry (such as bombs), launching aerial torpedo, torpedoes, or deploying air-launched cruise missiles. The first use of bombs dropped ...

aircraft, a single example of the

B-29 Superfortress 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 Fl ...

converted to fly with alternative powerplants. It was intended to demonstrate that the B-29 could still be put into service even if the first choice of

engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gen ...

, the air-cooled

Wright R-3350 The Wright R-3350 Duplex-Cyclone is an American twin-row, supercharged, air-cooled, radial aircraft engine with 18 cylinders displacing nearly . Power ranged from 2,200 to over 3,700 hp (1,640 to 2,760 kW), depending on the model. ...

radial engine The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders "radiate" outward from a central crankcase like the spokes of a wheel. It resembles a stylized star when viewed from the front, and is ca ...

, ran into development or production difficulties.

Design and development

Starting life as the first YB-29 delivered to the

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, it was sent in November 1943 to the

Fisher Body Fisher Body was an automobile coachbuilder founded by the Fisher brothers in 1908 in Detroit, Michigan. A division of General Motors for many years, in 1984 it was dissolved to form other General Motors divisions. Fisher & Company (originally Allo ...

Aircraft Development Section of

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to be converted to use

Allison V-3420 The Allison V-3420 was a large experimental piston aircraft engine, designed in 1937 by the American Allison Engine Company. Design and development In 1937, at the behest of the United States Army Air Corps, the Allison Engine Company agreed t ...

-17 liquid-cooled W24 (twin-

V12 V12 or V-12 may refer to: Aircraft * Mil V-12, a Soviet heavy lift helicopter * Pilatus OV-12, a planned American military utility aircraft * Rockwell XFV-12, an American experimental aircraft project * Škoda-Kauba V12, a Czechoslovak experim ...

, common crankcase) inline engines.''Flying Magazine,''August 1945, p. 51. Fisher was chosen for the modification as it was familiar with the engine, as it was to power the P-75 Eagle that they were then developing. Testing on it began in early 1944. Further development of the engine and the aircraft was delayed by a series of changes in the planned

turbosupercharger In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pro ...

s, as the originally specified GE Type CM-2 two-stage turbosupercharger became unavailable due to demands on GE's production of its other turbosuperchargers. Other turbosuperchargers were considered, but the end result was that the first flights of the B-39 had to be made without any turbosuperchargers at all.

Operational history

Fisher finally focused again on the B-39. The first flight of the B-39 was made on 9 December 1944 at Cleveland, Ohio. The initial flight tests of the B-39, without turbosuperchargers installed, were impressive. However, the B-39 program was by now seriously delayed, and the flawed R-3350 B-29s had already been rushed into combat in June 1944. Despite continuing problems with the B-29s, the aircraft was functioning well enough in combat that it no longer made any sense to shift resources in the manufacturing base to a new engine for the B-29 and so the B-39 was not ordered into production.

Specifications (XB-39)

References

Notes

Bibliography

* Jones, Lloyd S. ''U.S. Bombers, B-1 1928 to B-1 1980s''. Fallbrook, California: Aero Publishers, 1962, second edition 1974. . * Whitney, Daniel. ''Vee's For Victory!''. Atglen, Pennsylvania: Schiffer Military History, 1998.

External links

XB-39 on USAF Museum site
{{USAF bomber aircraft B-39 Superfortress Boeing B-39 Superfortress Four-engined tractor aircraft XB-39 Mid-wing aircraft Aircraft first flown in 1944 Four-engined piston aircraft