The Douglas GAM-87 Skybolt (AGM-48 under the 1962 Tri-service system) was an air-launched ballistic missile (ALBM) developed by the United States during the late 1950s. The basic concept was to allow US strategic bombers to launch their weapons from well outside the range of Soviet defenses, as much as 1,000 miles (1,600 km) from their targets. To do this in an air-launched form, a lightweight thermonuclear warhead was needed. Initially, the W47 from the Polaris missile was selected, but it was later replaced by the W59 from the Minuteman missile.