• Douglas PGM-17 Thor

The Thor was the first Intermediate Range Ballistic Missile (IRBM) deployed by the U.S. armed forces. Although its military career was relatively short, its descendants are still in use as space launch vehicles.
Development began in 1954 with USAF studies about a 2400 km (1500 miles) range ballistic missile to complement the long-range ICBM. Soviet ballistic missile progress resulted in a decision in 1955 to develop an IRBM, to be named Thor, as quickly as possible. Using existing components (the Rocketdyne S-3D liquid-fuel rocket motor from the Army's SM-78/PGM-19 Jupiter IRBM, and the inertial guidance unit and Mk.2 reentry vehicle from the SM-65D/CGM-16D Atlas), and requiring the missile to be air-transportable by C-124 Globemaster transport aircraft, the basic design and overall dimensions of the Thor were quickly determined. Go-ahead for development was given in September 1955, and in December 1955, Douglas was selected as prime contractor for the SM-75 Thor IRBM.