Concerns about the vulnerability of the B-52 to interception by Soviet defenses led to the creation of the Hound Dog missile in 1957. The cruise missile was designed to be launched from outside the range of Soviet air defenses and fly at Mach 2 at high altitude and to carry a four-megaton nuclear warhead. Each B-52 could carry two Hound Dogs on external pylons under the wings. A unique feature of the missile is that its jet engine could be used to provide extra thrust for the aircraft carrying it during takeoff, it would then be refueled from the bomber’s fuel tanks before launch. A total of 600 Hound Dogs were built. They served from 1961 to 1978.