The Boeing B-52 Stratofortress is an American long-range, subsonic, jet-powered strategic bomber. It has been operated by the United States Air Force (USAF) since the 1950s. The bomber is capable of carrying up to 70,000 pounds (32,000 kg) of weapons, and has a typical combat range of more than 8,800 miles (14,080 km) without aerial refueling. The B-52H had the same crew and structural changes as the B-52G. The most significant upgrade was the switch to TF33-P-3 turbofan engines. The ECM and avionics were updated, a new fire control system was fitted, and the rear defensive armament was changed from machine guns to a 20 mm M61 Vulcan cannon (later removed in 1991–94). The aircraft’s first flight occurred on 10 July 1960, and it entered service on 9 May 1961. This is the only variant still in use. A total of 102 B-52Hs were built. The last production aircraft, B-52H AF Serial No. 61-0040, left the factory on 26 October 1962.