Boeing XB-39 Superfortress

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. The aircraft was camouflaged in the standard USAAF Olive Drab over Neutral Gray scheme. Note the forward turret is equipped with only two .50 caliber machine guns instead of the later four-gun turret.
The Allison V-3420-17 was a 24-cylinder inline, basically made by joining two Allison V-1710 engines to a common crankcase. It was a successful design with a 1:1 power to weight ratio.
The Fisher Body division of General Motors was selected for the work due to their experience in developing the XP-75 Eagle long-range fighter, a design which also was to use the Allison V-3420. 41-36954 was christened “The Spirit of Lincoln” and conversion work began at Fisher’s Detroit plant.
Technicians are seen at work on the engine nacelle in this page from a Fisher’s brochure. Modelers should note that the Olive Drab paint has been worn off the wings around the new nacelles, which have been left in natural aluminum and that the leading edge fairings on the wings have been removed.
The result was an attractive aircraft with a hybrid finish of new natural aluminum components added to a camouflaged airframe.
The planned General Electric superchargers were not ready when the first flight was made on 09DEC44. In spite of the lack of superchargers, the XB-39 was able to surpass 400 mph, demonstrating the validity of the concept.
An interesting photograph of the undersides of the XB-39 in flight. The horizontal tail surfaces and rudder have been replaced, and the starboard aileron has had its outer surfaces removed revealing the internal structure. This was sometimes done to solve aerodynamic flutter but it is not clear if that was the reason in this case.
This photograph shows off the engine nacelles to good advantage.
Despite the success of the conversion, the decision was made to continue B-29 production using the R-3350 radials, as the issues with the radial engines were being rectified and supply chain issues did not materialize. The Spirit of Lincoln would remain the only B-39 ever constructed.