Development of long-range bombers, such as the B-29 Superfortress during World War II, was continued during the Cold War period. In 1946, the Convair B-36 Peacemaker became the first purpose-built nuclear bomber; it served with the USAF until 1959. The Boeing B-52 Stratofortress was able by the mid-1950s to carry a wide arsenal of nuclear bombs, each with different capabilities and potential use situations. Starting in 1946, the U.S. based its initial deterrence force on the Strategic Air Command, which, by the late 1950s, maintained a number of nuclear-armed bombers in the sky at all times, prepared to receive orders to attack the USSR whenever needed. This system was, however, tremendously expensive, both in terms of natural and human resources, and raised the possibility of an accidental nuclear war.