In 1966, the U.S. Air Force identified a need for an aeromedical transport aircraft and ordered C-9A Nightingale aircraft the following year. Deliveries began in 1968, and the C-9As were used for medical evacuation, passenger transportation, and special missions from 1968 to 2005. The C-9A aircraft were named for English social reformer Florence Nightingale (1820–1910), the founder of modern nursing.
After selecting a modified DC-9 for passenger and cargo transport, the U.S. Navy ordered its first five C-9Bs in April 1972. The C-9B aircraft have provided cargo and passenger transportation as well as forward deployed air logistics support for the Navy and Marine Corps. (The original "Skytrain" was the famous C-47 of the World War II era, developed from the civilian DC-3.) A C-9B was also chosen by NASA for reduced gravity research, replacing the aging KC-135 Vomit Comet.
Many of the Navy's C-9Bs have a higher maximum gross take-off weight of 114,000 lb (52,000 kg) and are fitted with auxiliary fuel tanks installed in the lower cargo hold to augment the aircraft's range to nearly 2,600 nautical miles (4,800 km) for overseas missions along with tail mounted infra-red scramblers to counter heat seeking missile threats in hostile environments.
The C-9 fleet was located throughout the continental U.S., Europe, and Asia.
- C-9A Nightingale - 23 aeromedical evacuation aircraft for the United States Air Force received from 1968.
- C-9B Skytrain II - 24 convertible passenger/transport versions for the United States Navy and Marine Corps delivered from 1973 to 1976. An additional five C-9s were converted from passenger configured DC-9s.
- VC-9C - 3 executive transport aircraft for the United States Air Force. The three aircraft were delivered to the US Air Force in late 1976.
- C-9K - 2 aircraft for the Kuwait Air Force.