Boeing-built Minuteman missile systems, operated by the Air Force Combat Command, are long-range, solid-fuel, three-stage, intercontinental ballistic missiles capable of carrying single or multiple nuclear warheads. The program, which began in 1958, is one of the company's longest military contracts. By April 1967, 1,000 Minuteman missiles were operational and installed in six sites across the country. At peak production, 39,700 Boeing people worked on Minuteman projects. The company built, installed and maintained the missiles in their silos and trained Air Force personnel involved in the program. Minuteman II, first launched in 1964, was capable of striking from six to eight targets with far greater accuracy than its predecessor. The next version, Minuteman III, was first launched in 1968, with more accuracy, range and target capability. By 1975, the 1,000 Minuteman U.S. missiles were either Minuteman IIs or IIIs. The current LGM-30G version has a top speed of more than 15,000 miles per hour and a range in excess of 6,000 miles.