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.