The Douglas DC-4E design originated in 1935 from a requirement by United Air Lines. The goal was to develop a much larger and more sophisticated replacement for the DC-3, before the first DC-3 had even flown.
There was enough interest from other airlines, that American Airlines, Eastern Air Lines, Pan American Airways, and Trans World Airlines joined United in providing $100,000 each toward the cost of developing the new aircraft. Pan American and TWA later withdrew their funds in favour of the Boeing 307 which was seen as being more economical.
With a planned capacity of 42 passengers, the DC-4 (as it was then known) would seat twice as many people as the DC-3. It would be the first large airplane with a nose wheel. Other innovations included auxiliary power units, power-boosted flight controls, alternating current electrical system, and air conditioning.
Cabin pressurization was also planned for the production aircraft. The aircraft also featured a novel tail with three vertical stabilizers. This provided sufficient vertical stabilizer area to allow the aircraft to take-off with only two engines on one side operating.
The prototype first flew on 7 June 1938 from Clover Field in Santa Monica, California. It was used by United Air Lines for in-service evaluation during 1939. Operating the aircraft was remarkably trouble-free. However, the complex systems proved to be expensive to maintain and the design was abandoned in favor of a smaller, less complex four-engined design. This newer design was designated DC-4, leading to the earlier design to be re-designated DC-4E (E for "experimental").
The DC-4E was sold to Japan, which was buying western aircraft for evaluation and technology transfer during this period. The design became the basis of the Nakajima G5N bomber.
