at a future date. BuNo 84030 and 84031 were later modified into anti-submarine patrol bombers and redesignated P2B-2S. XB-39 Superfortress Section source: see Baugher The XB-39 Superfortress was a single YB-29 modified to use water-cooled Allison V-3420-17 Vee type engines. Since the Army Air Force was concerned that problems might develop with their first choice of engine, the Wright R-3350, they contracted General Motors to test a modified aircraft to show that it could still be used if the R-3350 development was not successful. Since the R-3350 did not have significant enough problems to prevent its use, no XB-39s were ordered. Tupolev Tu-4 In 1945, three B-29s were forced to land in Soviet territory after a bombing raid on Japan because of lack of fuel. Since the Soviet Union was not at war with Japan at the time, the aircraft and crew were interned. Eventually, the B-29 crew members were returned, but the aircraft remained in Russian hands. Seeking a modern long-range bomber, Joseph Stalin ordered the Tupolev OKB to reverse-engineer the Superfortress. The resulting aircraft first flew on May 19, 1947, and immediately began series production, totalling 847 Tu-4s. Although largely identical in appearance to American B-29s, the Tu-4 (NATO reporting name: "Bull") had Soviet-designed defensive guns and had been re-engineered to suit production using material of metric thicknesses, resulting in an aircraft that was slightly heavier and slower than the B-29. The Tu-4 presented a significant leap forward in Soviet strategic bombing. Not only did the Soviet Air Forces have the means to deliver nuclear weapons, but the Tu-4 had sufficient range to reach the United States on a one-way trip. On October 18, 1951, a Tu-4 was used in the first air-drop test of a Soviet atomic bomb. The Tu-4 had been phased out of Soviet service by the early 1960s, being replaced by more modern aircraft such as the Tupolev Tu-95. Although the Tu-4 had never delivered any explosive payload with offensive intent, it influenced Soviet aircraft technology, particularly airframe construction and onboard systems. Advanced transport and bomber variants of the Tu-4 design such as the Tu-70, Tu-75, Tu-80, and Tu-85, were developed and built, but none of these achieved series production. The People's Liberation Army Air Force of China attempted to use the Tu-4 airframe in the KJ-1 AWACS aircraft. See also Boeing B-29 Superfortress B-29 Superfortress operators B-29 Superfortress survivors List of bomber aircraft