From old site on the Net,
in the autumn of 1943, as the first production B-29 "Superfortress" heavy strategic bomber began to enter service with USAAF training bomber units, Boeing began intensive work on modernizing the aircraft. One of the new components to be tested was the Allison V-3420 high-output 24-cylinder engine, whose design had begun at the request of the US Army Air Corps back in 1937. This engine was essentially a combination of two V-1710 12-cylinder in-line engines. It had a common crankcase and two crankshafts, driving the propeller shaft via a gearbox.
To test the new engine, the first YB-29-BW test aircraft (serial number 41-36954) was transferred to General Motors Corporation's Power-plant Development Division, whose engineers designed, built, and installed the entire new powerplant system on the airframe. The bomber, bearing the military designation XB-39-GM and nicknamed Spirit of Lincoln , underwent a series of flight tests, demonstrating that despite an increase in empty weight of nearly 3 tons compared to a standard B-29A or B-29B, the aircraft with the new engines had significantly improved performance while maintaining its combat capabilities.
However, despite its improved performance and lack of any side effects, the XB-39 failed to replace the B-29 bombers on the production line and remained in production with only a single prototype. The reasons were likely fourfold, and they are easy to surmise. The B-29, equipped with Wright R-3350 engines, had sufficient performance to meet the Air Force's current expectations. Furthermore, significant expenditures had already been made preparing for mass production of both the airframe and the engines. Furthermore, the USAAF urgently required ever-increasing numbers of bombers, and such a radical powerplant change (with the simultaneous lack of mass production of the V-3420 engines) was bound to adversely affect production rates. Finally, the XB-39's performance was not sufficiently superior to the B-29 to justify further massive expenditures.
The XB-39 was undoubtedly one of the best heavy bombers developed during World War II, but it remained in prototype form. After the war, the B-29 underwent a powerplant upgrade. After replacing the engines with 28-cylinder quadruple-radial Pratt & Whitney R-4360 "Wasp Major" engines, the XB-44 was developed, which was later put into series production under the designation B-29D, and ultimately B-50.