The Wright-powered B-29 had always been somewhat underpowered for its weight, and it became clear that the aircraft could take substantially more engine power if it were available. In pursuit of this objective, one B-29A (42-93845) was handed over to Pratt & Whitney in mid-1943 for conversion as a testbed for the new four-row 28-cylinder Pratt & Whitney R-4360 Wasp Major air-cooled radial engine, which was rated at 3500 hp. The aircraft was later redesignated XB-44, and was readily recognizable by the new engine installation, with the oil cooler intake pulled further back on the lower part of the nacelle.
The XB-44 was never intended to be anything but an engine testbed, but it did carry minimal defensive armament--a pair of 0.50-in machine guns in the tail.
The XB-44 made its first flight in May of 1945, and had a cruising speed of about 50-60 mph faster than that of the production B-29. Although the XB-44 had a superior performance to the B-29, the entire program was almost cancelled because of the approaching end of World War II. However, the USAAF felt that they would still need a very heavy bomber in the postwar era and lobbied for continuation of the program. An order for 200 production examples under the designation B-29D was placed in July of 1945, but was reduced to only 50 after V-J Day.
In December of 1945, the designation of the B-29D was changed to B-50A. This was a ruse to win appropriations for the procurement of an airplane that appeared by its designation to be merely a later version of an existing model that was already being cancelled wholesale, with many existing models being put into storage. Officially, the justification for the new B-50 designation was made on the basis that the changes introduced by the B-29D were so major that it was essentially a completely new aircraft. The ruse worked, and the B-50 survived to become an important component of the postwar Air Force.