The Boeing B-52 Stratofortress is a long-range, jet strategic bomber flown by the United States Air Force (USAF) since 1954.  The B-52 replaced the Convair B-36 and the Boeing B-47. Although built for the role of Cold War-era nuclear deterrence, its conventional capabilities now take priority. The aircraft has the longest range of any bomber and carries a heavy strategic or tactical weapons load.  Its economy in operation and high subsonic performance compared to the rest of the USAF strategic bomber fleet has enabled it to continue to serve rather than be replaced by the Mach 3 XB-70 Valkyrie program (that was cancelled), supersonic B-1B Lancer, and stealth B-2 Spirit. In January 2005, it was the second aircraft, after the English Electric Canberra, to mark 50 years of continuous service with its original primary customer.  The USAF's Strategic Air Command had B-52 Stratofortresses in active service from 1955 through 1991, when the aircraft were assigned to the Air Combat Command.