Douglas C-54 Skymaster
Other designations were DC-4, R5D. Douglas produced this four-engine transport about twice the size of the DC-3 and, in 1938. 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 a special C-54C, nicknamed the "Sacred Cow" by the White House press corps, became the first presidential aircraft, ordered for Franklin D. Roosevelt.

After World War II, commercial airlines placed more than 300 civilian DC-4 transports into service. DC-4s and used C-54s carried more passengers than any other four-engine transport. Some were still operating around the world through the year 1998!