In 1962, the German Federal Ministry of Defense awarded Dornier a design contract for the Do 31 V/STOL transport aircraft. Under this experimental program, the production program covered a small and a large hovering rig for studying design principles, an airframe for structural testing, and a systems test stand for hydraulic and electric systems. Two prototypes were built and a third airframe was completed for static tests.
The Do 31 E-1 was equipped with two engines providing power for cruising flight as well as lift during takeoff and landing via vectored nozzles. To support the cruise engines in hover flight another eight engines were installed in nacelles at the wing ends. By tilting the cruise engine nozzles, the Do 31 was accelerated to the speed of approximately 250kph required for aerodynamic horizontal flight, and the eight lift-producing engines were stopped again after 20 seconds.
In the two prototypes, the main cabin was occupied by test equipment, but the fuselage volume of 1,765 cu ft (50.0 cu.m) would be equivalent, in a production version, to accommodation for 36 people. The Do 31 E was preceded by a small -flying bedstead test rig, which in its one year's flying made 247 test flights with nine different pilots; and then, in January 1967, by a larger rig which immediately preceded the first flight of the Do 31 E1 on 10 February 1967.
The first Do 31E1 first flew on 10 February 1967 with two Pegasus vectored-thrust turbofans in underwing nacelles and two removable wingtip pods each containing four 4400-lb (1996-kg) thrust RB.162 turbojets.
During its three-year test programme, the Do 31 investigated problems associated with all-weather flying and operational noise levels, as well as those concerned with VTOL operation.
The Do 31, which established several world records during its ferry flight to the 1969 Paris Air Show.
The Do 31 E concluded its test flying programme in April 1970, exactly six years after Dornier's first little test rig made its original flight.
Engines: Two 15.500 lb (7,000 kg) st Rolls-Royce Bristol Pegasus 5-2 vectored-thrust turbofan for propulsion, plus eight 4,400 lb (2.000 kg) st Rolls-Royce RB.162-4D lift-jets in wing- tip pods.
Wing span: 59 ft 3 in (18.06 m).
Length: 68 ft 6 in (20.88 m).
Height: 8.53 m / 27 ft 12 in
Wing area: 57.0 sq.m / 613.54 sq ft
Gross weight: 60,500 lb (27,500 kg).
Empty weight: 22453 kg / 49501 lb
Max. cruising speed: 400 mph (650 km/h) at 20,000 ft (6,000 m).
Ceiling: 10515 m / 34500 ft
Accommodation: up to 36 fully-equipped troops.