Air Force originally ordered 5,000, but cancelled its request when World War II ended and none were built. B-29D (XB-44) Section source: Baugher The B-29D was an improved version of the original B-29 design, featuring 28-cylinder Pratt & Whitney R-4360-35 Wasp Major engines of 3500 hp (2600 kW) each — nearly 60% more powerful than the usual Duplex-Cyclone. It also had a taller vertical stabilizer and a strengthened wing. The XB-44 was the testbed designation for the D model. When World War II ended, the B-29D was given the quartet of Wasp Major engines to become the B-50, which served throughout the 1950s in the U.S. bomber fleet. Test beds A number of B-29s were converted to serve as test beds for new systems. These all received variant designation, even though many existed only as a single converted aircraft. XB-29E The XB-29E for fire-control systems (one converted) was a model B-29-45-BW. XB-29F The B-29F for cold-weather operation in Alaska were six converted B-29-BWs. XB-29G The B-29 was used in the development of jet engines. Stripped of armament, a converted B-29B-55-BA (44-84043)(Bell) designated the XB-29G carried experimental jet engines in its bomb bay, which were extended into the airstream for testing during flight. This plane was used to test the Allison J35, General Electric J47 and J73 jet engines. XB-29H The XB-29H to test armament configurations was a converted B-29A. YB-29J, YKB-29J, RB-29J Experimentation in piston engines continued. Six B-29s (redesignated YB-29J) of various designation were upgraded to R-3350-79 engines. Other engine-associated items were also upgraded, including new Curtiss propellers, and 'Andy Gump' cowlings, in which the oil coolers have separate air intakes. Two were later converted to aerial refueling tanker prototypes, and redesignated YKB-29J. The remainder were used for reconnaissance, and designated RB-29J. KB-29 Tanker Development EB-29 Section source: Baugher The EB-29 (E stands for exempt), was used as a carrier aircraft in which the bomb bay was modified to accept and launch experimental aircraft. They were converted in the years following World War II. One EB-29 was converted to carry the famous Bell X-1 until it was replaced by a B-50. Another was used to carry and test the XF-85 'parasite fighter'. This fighter was intended to be carried by the Convair B-36 on long-range missions to protect it from Soviet fighters. Another EB-29 was used to carry two EF-84B Thunderjet fighters as part of Project Tom-Tom. All three