W.T. LARKINS COLLECTION
No. 2972. Boeing 345 XB-39 (41-36954 c/n 3325) US Army Air Forces
Photographed by W.T. Larkins
08/31/2010. Remarks by Johan Visschedijk
: "The first YB-29 (later redesignated XB-39) was turned over to General Motors for use as a test bed for new Allison V-3420 24-cylinder liquid-cooled V-engines. These were essentially pairs of the standard Allison V-1710 fighter engines combined on a common propeller shaft to deliver 2,600 take-off horse power. Normal output was 2,100 hp at 25,000 ft (7,620 m), but the engines were not ready for production. While aircraft speed increased to 405 mph at 35,000 ft (10,668 m), overall performance was not sufficiently improved over the existing air-cooled R-3350s to justify further conversions."