They'are both the same.
When it was decided that the President of the United States, Franklin D. Roosevelt, should have a personal airplane to transport him to meetings around the world, the aircraft of choice was the US Army Air Corps’ (USAAC) C-54A Skymaster. Dubbed ‘The Sacred Cow’, this was the plane that took the president to Tehran, Casablanca, Hawaii and other less exotic spots in the USA.
The C-54 was the military derivative of the Douglas DC-4, a four-engine long range airliner with a three-man crew and accommodations for up to 49 passengers or 26 troops. Originally designed to a specification from United Airlines, the DC-4 had a maximum speed of 274 mph and a range of 3900 miles. The first 61 civilian orders were followed by a further buy for 71 from the USAAC though, in the end, most ended up in Army service.
To meet the military’s more stringent needs, the DC-4 was given a cargo door, stronger floor, cargo boom hoist and larger wing tanks. First flight of the military C-54 occurred on 26 March 1942. During the war years, 1242 C-54s were delivered with a wide variety of modifications. A few of the major ones were the C-54A, the original, fully militarized model capable of lifting 50 soldiers or 32,500 pounds of cargo; the JC-54D, which was modified for missile nose cone recovery; the C-54E, with larger Pratt & Whitney engines, bigger fuel tanks for longer range and a specially designed cabin for quick conversion between passenger and cargo roles; the C-54M, which was a C-54E stripped out to serve as a coal-carrier during the Berlin Airlift; the EC-54U, a post-war modification as an electronic counter-measures platform; and at least 14 sub-variants built for the US Navy originally called the R5D. There were numerous other variants which performed countless other roles, from VIP transport to multi-engine training.
The C-54 offered sterling service for both the USAF and the US Navy after the war, and was not fully retired until the late 1960’s. Ex-military Skymasters became popular as cargo transports and fire bombers, and many are still in active use around the world in these roles. A lucky few have been acquired by appreciative warbird groups in the USA.
