The McDonnell Douglas C-9 was a military version of the McDonnell Douglas DC-9 airliner. It was produced as the C-9A Nightingale for the United States Air Force, and the C-9B Skytrain II for the U.S. Navy and Marine Corps. The final flight of the C-9A Nightingale was in September 2005, and the C-9C was retired in September 2011. The U.S. Navy retired its last C-9B in July 2014. The two remaining C-9s in Marine service were retired in April 2017.