The DC-4 program began when United Airlines
requested a new airliner from Douglas
with a higher capacity than the Douglas DC-3
Douglas delivered two gigantic prototype DC-4E's in 1938, with a split-level deck, pressurization, and a triple tail like the Lockheed Constellation's. United experimented with one of these, but decided that it wasn't economical enough, and sent it back in 1939 with a request for something simpler. Japan bought the plane and reportedly used it in studies for a long-range bomber program.
Douglas redesigned the DC-4 with a single deck, four Wasp piston engines, and a more orthodox wing and tailplane design. United, American Airlines, and Pan American World Airways placed firm orders, and the United States Army also ordered the type as the C-54 Skymaster. By the time the first DC-4's were coming off the line, World War II was underway, so the DC-4 never saw civilian service until 1946.
In the meantime, the C-54 was showing its colors as an airlift powerhouse. They were most revered for their role in the Hump route that kept the Kuomintang supplied, and later became famous in the Berlin Airlift as well. Franklin D. Roosevelt and Harry S. Truman used C-54's as the first presidential aircraft.
1,245 DC-4's were built in total. Most started life as USAAF C-54's, but 79 civilian aircraft with pressurization and other comforts were also delivered to various airlines to add to the widespread fleets of refitted military aircraft. The DC-4's range opened up a wide variety of routes that were previously accessible only to seaplanes, including Atlantic crossings, Pacific crossings, and South Pacific routes. The DC-4 made it possible to fly from New York to LA in just 14 hours, as opposed to 18 hours by DC-3. Soon, the Douglas DC-6 overshadowed the DC-4, but in the late forties and early fifties, the DC-4, Constellation, and Boeing 307 were the pinnacle of air travel.
Two variants of the DC-4 were also built. The Canadair North Star was a Canadian-built DC-4 with Rolls Royce Merlin engines for higher speed and range. Later, Freddie Laker of Laker Airways fame had a few DC-4's expanded to be able to ferry cars, resulting in the Carvair.
Several DC-4's still fly today. Some are still used for airline service in the far north of Canada and Alaska: a few more have been refitted as water bombers. There are scores of museum specimens enshrined around the world as well.
The DC-4 weighs 66,000 pounds fully loaded, and cruises at up to 200 knots. It carries 44 passengers in two classes, or 80 in one class, with a range of 3,500 miles.