President Harry S. Truman signed the National Security Act of 1947, which created the U.S. Air Force, on board "Sacred Cow", the Presidential VC-54C which is preserved at the National Museum of the United States Air Force near Dayton, Ohio. More than 300 C-54s and R5Ds formed the backbone of the US contribution to the Berlin Airlift in 1948. They also served as the main airlift during the Korean War. After the Korean War, the C-54 was replaced by the Douglas C-124 Globemaster II, but continued to be used by the U.S. Air Force until 1972. The last active C-54 Skymaster in U.S. Navy service (C-54Q, BuNo "56501", of the Navy Test Pilot School, NAS Patuxent River) was retired on 2 April 1974.