In 1955 the USAF started to develop an Intermediate Range Ballistic Missile (IRBM). The project, designated 315A, was given top priority. In December 1955, the Douglas Aircraft Company was awarded the prime contract. The 315A was re-designated SM-75 THOR and delivered to USAF in October 1956.
The system was declared operational in 1959 and was designed to deliver a nuclear payload.
In the UK, the first Missile became operational at RAF Feltwell in December 1959, and within RAF Bomber Command in April 1960. By 1961 there were twenty Thor Missile squadrons in the UK, each having three Missiles. The Thor Missile was withdrawn from service in 1963.