The C-54 began as a late 1941 redesign of the DC-4 to meet US Army Air Force specifications, and was ordered in large quantities without a prototype.
Following the Bombing of Pearl Harbor, which led to the entry of the United States of America into World War 2, the civilian DC-4 airliners already on the production line were finished with military markings and drab paint schemes, with the first of these flying on 14 February 1942. This led to production contracts for militarised versions, ca[able of deployment for transportation of troops, cargo and casualties. Construction for the Air Force and the Navy exceeded 1,000 examples
- C-54: Initial military conversion of the DC-4. Lacked cargo door, heavy duty floor and floor support structure found on later variants. Four 1,100hp Pratt & Whitney R-2000-3. Fuel capacity 3,580 US gallons.
- C-54A: Redesign for carriage of cargo and/or passengers. Four 1,100 R-2000-7. Fuel capacity 3,620 US Gallons. Fifty six operated by US Navy as R5D-1.
- C-54B: C-54A with modified fuel system - two fuselage fuel tanks removed and replaced by outer wing tanks - and standardised cabin fittings to permit rapid change from cargo to troop transport, including individual oxygen outlets and removable stretcher fittings. Fuel capacity 3,720 US Gallons. Thirty operated by US Navy as R5D-2.
- VC-54C: Special aircraft modified for personal use by President Roosevelt, with electrically operated elevator, four staterooms - including one for the President - and main cabin with conference table.[N 3]
- C-54D: Cargo version with C-54B style interior. Produced in Chicago plant only. Ninety five operated by US Navy as R5D-3.
- C-54E: Improved C-54-D, combining features of original C-54 with those of C-54A and C-54B. Fuel capacity 3,540 US Gallons. Produced in Santa Monica plant only. Twenty operated by US Navy as R5D-4. [N 4]
- C-54F: Modified C-54Bs incorporating special requirements of Troop carrier Command.
- C-54G: C-54E with R-2000-9 engines. Thirteen operated by the United States Coast Guard as R5D-5.
- C-54J: C-54G with full airline furnishings for personnel transport. No cargo facilities. Known to US Navy as R5D-6. 
- One C-54B and 22 C-54D aircraft were passed to the RAF under Lend-Lease as the Skymaster I, with at least eleven examples serving with 232 Squadron during the final months of the war.
- This is the designation for US Army Air Force aircraft. United States Navy examples used the R5D designation.
- This aircraft received the name Sacred Cow.
- Many of the C-54E's improvements, including fitment of R-2000-11 engines, were tested using C-54Ds.
- Image of C-54 in flight via Wisbech Aero Club
- World Aircraft Information Files Aviation Partwork. Midsummer Books Ltd. File 892 Sheet 40 (A-Z of aircraft:D - Douglas DC-1 and DC-2 (continued) to Douglas DC-4E, DC-4, C-54 Skymaster and R5D
- Fly Past. Published monthly by Key Publishing Limited June 2014 Page 111
- Gunston, Bill (Forward). Jane's Fighting Aircraft of World War II. Tiger Books. 1989. ISBN 1-85501-996-5. (Reprint of Jane's All The World's Aircraft 1945/1946. Bridgeman, Leonard (Editor). 1946). Page 224
- Gunston, Bill. 1989. Page 225