High Quality Content by WIKIPEDIA articles! The McDonnell Douglas C-9 is a military version of the 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. Naval Reserve and Marine Corps. The final active-duty flight of the C-9A Nightingale was in September 2005. The Boeing C-40 Clipper is replacing the Navy Reserve`s aging C-9B fleet.