The massive B-52 was the U.S.'s long-range strategic bomber in the 1950s, used to extend the range of nuclear weapons worldwide. The Pentagon sought an eventual replacement, a bomber that could cruise at ultra-high altitude, carry nuclear bombs, and stay aloft for extended periods. It also needed extreme speed to escape intercepting fighters, the primary threat to long-range bombers at the time. Since no one thought ordinary jet engines could produce enough power efficiently enough to meet all those design requirements, early research focused on installing a nuclear reactor in a plane.