The Douglas DC-4 is an American four-engined (piston), propeller-driven airliner developed by the Douglas Aircraft Company. Military versions of the plane, the C-54 and R5D, served during World War II, in the Berlin Airlift and into the 1960s. From 1945, many civil airlines operated the DC-4 worldwide.
Following proving flights by United Airlines of the DC-4E, it became obvious that the 52-seat airliner was too inefficient and unreliable to operate economically and the partner airlines, American Airlines, Eastern, Pan American, Trans World and United, recommended a lengthy list of changes to the design. Douglas took the new requirements and produced an entirely new, much smaller design, the DC-4A, with a simpler, still unpressurized fuselage, Pratt & Whitney R-2000 Twin Wasp engines, and a single fin and rudder. A tricycle landing gear was retained.
With the entry of the United States into World War II, in December 1941, the United States Army Air Forces took over the provisional orders for the airlines and allocated them the designation C-54 Skymaster. The first C-54 flew from Clover Field in Santa Monica, California, on 14 February 1942.
To meet military requirements, the first production aircraft had four additional auxiliary fuel tanks in the main cabin, which reduced the passenger seats to 26. The following batch of aircraft was the first built to military specifications, and was designated C-54A and built with a stronger floor and a cargo door with a hoist and winch. The first C-54A was delivered in February 1943. With the introduction of the C-54B in March 1944, the outer wings were changed to hold integral fuel tanks, allowing two of the cabin tanks to be removed; this allowed 49 seats (or 16 stretchers) to be fitted. The C-54C was a hybrid for Presidential use, it had a C-54A fuselage with four cabin fuel tanks and the C-54B wings with built-in tanks to achieve maximum range.
The most common variant was the C-54D, which entered service in August 1944, essentially a C-54B with more powerful R-2000-11 engines.