The Douglas C-47 Skytrain was the first fully militarised transport to be based on the DC-3 airliner, and was the first transport aircraft to be ordered in large numbers for the USAAF. Eventually 965 C-47s were built, before production moved on to the C-47A.
A number of modifications had to be made to turn the DC-3 into a useful military transport. The standard airline passenger door had to be replaced with a large split cargo door, hinged at both ends (unlike the door on the DC-2 based C-33, which was hinged at the front and in the middle). The front half of the door on the DC-3 contained a smaller conventional passenger door, which could be opened independently of the main doors. A cargo hoist could be installed above the door, to allow heavier cargos to be loaded.
Inside the aircraft the airliner interior was stripped out, and replaced with 28 folding canvas seats mounted on the side of the fuselage. These were eventually replaced with 28 folding metal seats, also attached to the side of the aircraft. The C-47 was also given a reinforced cargo floor, allowing it carry up to 6,000lb of cargo. Cargo tie-down rings were installed in the floor.
The C-47 carried 804 US gallons of fuel, a slight decrease on the 822 gallons of the DC-3, but another 900 gallons could be added in fuselage tanks to increase the ferrying range of the aircraft. Finally the wingspan was increased by 6 inches over that of the commercial DC-3.
The first order for the C-47 (for 545 aircraft) was placed in September 1940, but the first aircraft were not delivered until two weeks after the Japanese attack on Pearl Harbor. By then thousands more aircraft had been ordered, and the C-47 Skytrain would remain the most important Allied transport aircraft to the end of the Second World War.
Despite its later fame, the DC-3 was not particularly well suited to act as a cargo plane. Even with the large cargo door it was not easy to manoeuvre heavy payloads, and the overall payload was itself too light. The cargo door was too far above the ground for the aircraft to be loaded from the bed of a truck, and when the aircraft was on the ground the cargo compartment was on a slope. It became so dominant because the Curtiss C-46 Commando and Douglas C-54 Skymaster both failed to appear in significant numbers until 1944.