The B-47 was the United States Air Force's first strategic jet bomber. When the US Army Air Forces issued a requirement for a jet bomber in 1944, four manufacturers presented proposals. It was Boeing's design for the B-47 that won for a number of reasons, but especially because it was capable of carrying the outsized nuclear weapons of the day. The B-47 became the cornerstone of America's nuclear deterrent force until the B-52 came into the inventory. At the peak of its career in 1956, 1,367 B-47s were in Strategic Air Command's (SAC) inventory of 1,650 bombers. The B-47 proved to be as fast as many of the jet fighters of the day, and its operational altitude was as high as 40,000 feet. The design for the B-47 was extremely successful, and was later adapted to the B-52 bomber and the KC-135 tanker, which later formed the basis for the Boeing 707. In fact, almost all jet-powered passenger airliners today can trace their design ancestry back to the B-47.