he Boeing B-52 Stratofortress is a long-range, subsonic, jet-powered strategic bomber operated by the United States Air Force (USAF) since 1955. Beginning with the successful contract bid on 5 June 1946, the B-52 design evolved from a straight-wing aircraft powered by six turboprop engines to the final prototype YB-52 with eight turbojet engines. The aircraft first flew on 15 April 1952 with "Tex" Johnston as pilot. Built to carry nuclear weapons for Cold War-era deterrence missions, the B-52 Stratofortress replaced the Convair B-36. Although a veteran of a number of wars, the Stratofortress has dropped only conventional munitions in combat. The B-52 carries up to 70,000 pounds (32,000 kg) of weapons. The USAF has possessed B-52s in active service since 1955. The bombers flew under the Strategic Air Command until SAC was disestablished in 1992 and its aircraft absorbed into the Air Combat Command (ACC). Superior performance at high subsonic speeds and relatively low operating costs have kept the B-52 in service despite the advent of later aircraft, including the Mach-3 XB-70 Valkyrie, the supersonic B-1B Lancer, and the B-2 Spirit. In January 2005, the B-52 became the second aircraft, after the English Electric Canberra, to mark 50 years of continuous service with its original primary operator. (As of 2009, the list has added the Tupolev Tu-95, the C-130 Hercules, the KC-135 Stratotanker, and the Lockheed U-2.