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The German Dornier Do 31 project was begun in the early 1960s as a 50,000 lb gross weight vertical take-off and landing military transport plane, capable of lifting 6,000-8,000 lb. It was 68 ft long, with a two pilot crew sitting side by side. It could load 1,470 cubic ft of cargo through the rear loading ramp. It used two 15,500 lb thrust Bristol Pegasus 5-2 engines and eight 4,400 lb thrust Rolls-Royce RB.162-4D engines. The powerplants were divided into four wing-mounted pods - a Pegasus pod and a pod of four lift engines on either side of the fuselage. The lift engine pods were located at the ends of the 59 ft span wing. The lift engine exhaust could be vectored backward or forward 15° for take-off and landing, respectively. The Pegasus could vector exhaust from 30° forward to 80° back. Differential vectoring and thrust levels were used for control in roll and yaw; pitch was affected by a puffer jet in the tail. After almost four years of hover rig tests of increasing size and realism to develop the autostabilization controls, three Do 31 aircraft were built: one each for conventional flight trials, ground testing, and hover and transition research.
The first aircraft made a conventional flight without the lift engine pods on 10 February 1967. The third aircraft made the first hover on 22 November 1967. First transition from vertical was on 16 December, and first transition to vertical 6 days later. It continued to fly until it was canceled in April 1970: the large drag and weight of the engine pods reduced the useful payload and range compared to contemporary conventional transports.
Source: AHS V/STOL Wheel
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