The Douglas C-54 (designated R5D by the U.S. Navy) was the military variation of the DC-4 four-engine commercial transport. It was the first four-engine transport to enter USAAF service. The USAAF accepted a total of 1,164 Skymasters from 1942 to 1947. Its maximum load capacity was 28,000 pounds of cargo or 49 passengers.
Although it served with the USAAF as a transport, the C-54 made history when it became the first "official" presidential transport aircraft (Air Force One). Known as "The Sacred Cow," it was built in 1944 for use by President Franklin D. Roosevelt. One special feature was an elevator behind the passenger cabin to lift the President in his wheelchair in and out of the plane. The passenger compartment included a conference room with a large desk and bullet-proof picture window. President Roosevelt made his first and only flight in this aircraft traveling to Yalta, in the USSR, in February 1945. For security reasons, the tail number on the aircraft was changed for this flight. After Roosevelt's death in April 1945, the "Sacred Cow" remained in presidential service during the first 27 months of the Truman Administration. On 26 July 1947, President Truman signed the National Security Act of 1947 while on board the "Sacred Cow." This act established the Air Force as an independent service, making the "Sacred Cow" the "birthplace" of the U.S. Air Force. It was later assigned to other transport duties and was eventually retired in October 1961.
During the Berlin Airlift in 1948, every C-54 the USAF had was pressed into service to supply the isolated city. Many C-54s were later converted into litter-carrying planes for use during the Korean Conflict, returning 66,000 patients to the United States.
In later years, Douglas developed the XC-112, a pressurized version of C-54E Skymaster military transport. It had a longer fuselage, larger rectangular windows in place of circular portholes, and four 2,100 hp Pratt and Whitney R-2800-34 radial engines. After testing, this aircraft entered commercial service as the DC-6 and military service as the C-118 Liftmaster.