The C-54 Skymaster transports were developed from the design of the civilian airliner DC-4; the naval version was designated R5D. They began service in 1942 with room for 26 passengers. Later in the war, the capacity was increased to 50 passengers. After the war, they were used as the primary transports of the United States Air Force, participating in early Cold War events such as the Berlin Airlift and the Korean War. 1,170 were manufactured during its career.
Douglas decided to produce a four-engine transport about twice the size of the DC-3 and, in 1938, developed the single DC-4E to carry 42 passengers by day or 30 by night. It had complete sleeping accommodations, including a private bridal room.
It proved too expensive to maintain, so airlines agreed to suspend development in favor of the less complex DC-4, but it was not put into commercial service until 1946. Its military derivative was the C-54 "Skymaster" transport, ordered by the U.S. Army Air Forces in 1942.
Douglas built 1,241 of the DC-4s and its military counterparts, including the R5D for the Navy. During the war, C-54s flew a million miles a month over the rugged North Atlantic -- more than 20 round trips a day. A special VC-54C, nicknamed the "Sacred Cow" by the White House press corps, became the first presidential aircraft, ordered for Franklin D. Roosevelt.
In the years immediately following the war, new DC-4s and used C-54s carried more passengers than any other four-engine transport. Some were still flying through 1998.
After World War II, commercial airlines placed more than 300 civilian DC-4 transports into service.