The McDonnell Douglas F-4 Phantom II was destined to become one of the finest weapons the Navy had ever used. It was first conceived as an attack aircraft, but was redesigned to fill the role of a long range high altitude interceptor using air-to-air missiles. Deliveries to operational units began in 1960, and by 1966 twenty nine Navy and Marine squadrons were flying the Phantom. The primary role for the early Phantoms had been interception and air superiority, but deployment of the type to Vietnam put emphasis on the secondary strike role, carrying a wide range of external stores. The F-4S variant was an F-4J model upgraded with structural strengthening, plus the provision for the later installation of leading-edge maneuver slats as used on the USAF F-4E. (source: United States Navy Aircraft since 1911 by Gordon Swanborough and Peter Bowers)