The Boeing XB-39 Superfortress was a United States prototype bomber aircraft, a single example of the B-29 Superfortress converted to fly with alternative powerplants.

XB-39超級堡壘轟炸機 （英語： Superfortress)是一架 波音 為測試 B-29超級堡壘轟炸機 使用新型發動機的可能性而研製的轟炸機。 其目的是證明，即使首選引擎 萊特 R-3350 （英语：Wright R-3350 Duplex-Cyclone） 星型引擎 遇到開發或生產困難，B-29仍然可以投入使用。

2020年7月29日 · Page details technical specifications, development, and operational history of the Boeing XB-39 (Spirit of Lincoln) Heavy Strategic Bomber Prototype Aircraft including pictures.

XB-39超级堡垒轰炸机 （英语： Superfortress)是一架 波音 为测试 B-29超级堡垒轰炸机 使用新型发动机的可能性而研制的轰炸机。 其目的是证明，即使首选引擎 莱特 R-3350 （英语：Wright R-3350 Duplex-Cyclone） 星型引擎 遇到开发或生产困难，B-29仍然可以投入使用。

The XB-39 Superfortress was a single YB-29 modified to use water-cooled Allison V-3420 -17 Vee type engines. Since the Army Air Force was concerned that problems might develop with their first choice of engine, the Wright R-3350, they contracted General Motors to test a modified aircraft to show that it could still be used if the R-3350 ...

2022年1月12日 · The Boeing XB-39 was an experimental project to evaluate the substitution of the Allison V-3420-17 inline engine for the Wright R-3350 radials on a B-29 airframe, should the R-3350 run into insurmountable engineering or supply issues. The airframe selected was the first YB-29 developmental aircraft, 41-36954.

XB-39 was a single example of the B-29 converted to fly with alternative powerplants. The aircraft was a modification of the first YB-29. Similar to the XB-38, the original Wright R-3350-21 piston radial engines were replaced with liquid-cooled Allison V-3420-11 V-engines. Only one plane was built, which was tested in 1944 by the USAAF.

The XB-39 would be fitted with Allison V-3420-17 liquid-cooled W24 inline engines. The project test bed was YB-29, S/N 41-36954; it was was transferred in November of 1943 to the Fisher Body Aircraft Development Section of General Motors to be fitted with the Allison engines.

Boeing-GM XB-39 Superfortress/41-36954. [Source: Unknown] 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.

As shipped, the XB-39 contains 4 Allison V-3420-11 liquid-cooled engines, each producing 2,100 horsepower, propelling the aircraft to a speed of 405 mph, with a range of 6,300 miles and a service limit of 100 kilometers /h can bring 35,000 feet.