Count the engines on a Dornier Do 31 and you get to ten. Two of them are Pegasus turbofans, the same vectored-thrust engines that made the Harrier work. The other eight do nothing whatsoever in cruise. They sit in pods on the wingtips, four a side, pointing at the ground, and they exist for one purpose: to hold a 22-tonne transport aircraft in the air while it has no forward speed at all.

It flew in 1967. It set world records in 1969. It was cancelled in 1970. And nearly sixty years later it is still the only jet-powered VTOL transport aircraft that has ever flown — a category with exactly one member, opened and closed by the same aeroplane.

Why West Germany Wanted This

The Do 31 makes no sense at all until you understand what the Luftwaffe was afraid of in 1960.

West Germany was the ground on which a NATO–Warsaw Pact war would be fought, and every planner on both sides assumed the same opening move: within hours, every runway in the country would be cratered or irradiated. An air force tied to concrete would cease to exist before it had flown a sortie.

The German response was dispersal — get the aircraft off the airfields and onto autobahns, clearings and improvised strips. That produced the zero-length launch trials, in which Luftwaffe F-104 Starfighters were fired off stationary ramps by rocket booster and recovered onto short strips using carrier-style arrester gear. It produced the EWR VJ 101 supersonic VTOL fighter. And under NATO requirement NBMR-4, it produced a demand for a transport that could resupply those dispersed sites without a runway of its own.

Dornier had been sketching VTOL concepts informally since 1959, formalised the design in 1961 under a team at Friedrichshafen led by Gustav Wieland, and received a government development contract in February 1962.

Inside the cockpit of the surviving Do 31, with a walkthrough of how the ten-engine layout was actually operated.

Ten Engines and Sixteen Fountains

The engineering problem was not lift. Lift is easy if you are willing to bolt on enough engines. The problem was everything lift causes.

Hovering a large aircraft on jet thrust generates what the engineers counted as sixteen separate fountains of gas rebounding off the ground, twelve of them hot. An engine that swallows its own exhaust loses thrust immediately, and an aircraft hovering at a thrust-to-weight ratio barely above 1.0 has nothing to spare. Dornier eventually solved it by angling the nozzles to 85 degrees rather than a true 90 — enough to push the hot gas slightly away from the intakes on takeoff. Curiously, landings never showed the problem at all.

They bled air from the Pegasus engines into the lift jets to help them start evenly. They ran dedicated studies on how the exhaust eroded the ground beneath. They worried about noise, because the airframe’s critical frequency sat uncomfortably close to what the lift engines produced naturally.

And they built a purpose-made flight control test rig, plus a hybrid computer — the Dornier DO-960 — simply to solve the differential equations describing how the thing would behave. The philosophy they landed on is telling: despite hovering like a helicopter, the Do 31 was flown like an aeroplane.

The Only Man Who Ever Did It

E1 flew first, on 10 February 1967, with only the two Pegasus fitted — a conventional aeroplane, testing conventional flight. E3, with all ten engines, made the first hover in July 1967. In December came the hard part: transitions, forward and back, between hovering and wingborne flight. On 28 February 1968 the aircraft managed multiple transitions in a single flight.

Dornier’s chief test pilot Drury Wood flew all of it. As confidence grew he began exploring what the aircraft would tolerate, including flying it deliberately backwards to prove that it could, and rolling it on another occasion.

He is still right about the second half of that sentence, and nothing has happened since to make him wrong about the first.

Wood was not a novice being handed a curiosity. He had flown Corsairs off USS Bennington over Okinawa, bombed Tokyo, fought at Inchon and the Chosin Reservoir, graduated from the US Navy Test Pilot School in 1952 and stayed on as an instructor — where he taught John Glenn. He worked alongside Chuck Yeager at Edwards before Dornier hired him in 1964. By the end of his career he had flown more than 150 types.

He died in 2019, aged 95, still holding them.

Period newsreel footage of the Do 31 in 1969.

Munich to Paris, 27 May 1969

Dornier had commercial ambitions. The company saw a civil VTOL transport in the design, talked to Douglas Aircraft and to Ling-Temco-Vought about partnership, and at one point turned a Douglas approach down.

To make the case publicly, they flew a Do 31 to the 1969 Paris Air Show. The ferry flight itself, Munich to Paris on 27 May, set a clutch of Fédération Aéronautique Internationale world records in Class H for VTOL aircraft — speed, altitude, distance and duration. Those records were set by an aircraft going to an air show, which tells you how little competition the category had.

Beaten by Arithmetic and Helicopters

The Do 31 was not cancelled because it failed. It hovered, it transitioned, it set records, and it never killed anybody. It was cancelled because the numbers underneath it never improved.

Eight lift engines and their pods are pure penalty in cruise: weight to carry, drag to push, volume that could have been fuel or freight. Every kilogram of that reduced the payload and range the aircraft could offer against a conventional transport of the same size. A VTOL transport that carries meaningfully less than a C-160 and cannot fly as far is a difficult thing to justify, however impressive the party trick.

Then there was the alternative nobody could argue with. While Dornier was solving exhaust re-ingestion, the heavy-lift helicopter grew up. A CH-53 or a Chinook carried a useful load point to point, needed no transition, burned far less fuel doing it, and already existed in numbers.

Dornier had answers on the drawing board — delete the outer nacelles and replace them with larger RB153 turbofans once available, or the proposed Do 131 with twelve or fourteen lift jets. None was built. According to Andrew Dow’s history of the Pegasus, what actually killed the programme was political: Bonn grew frustrated that no other NATO country would commit money, and was not prepared to fund full-scale development alone.

The announcement came in April 1970. The Do 31 made its final public flight on 4 May 1970 at the ILA in Hannover, and that was the end of the category.

A full documentary on the programme, from NBMR-4 through to cancellation.

A Door That Stayed Shut

The Luftwaffe abandoned VTOL entirely — the VJ 101, the Do 31 and the later VFW VAK 191B all ended as research programmes, and West Germany never fielded a vertical-takeoff combat aircraft.

The requirement did not vanish, though. The idea of moving cargo into places without runways is exactly what the V-22 Osprey now does with tiltrotors, and what a generation of electric VTOL designs is chasing at smaller scale. Both use rotors. Nobody has gone back to pure jet lift for a transport, because the fuel arithmetic that beat Dornier has not improved in sixty years.

What Dornier proved was that it could be done. What Dornier also proved, more expensively, was that doing it and being worth doing are separate questions.

Both flying prototypes survive. E1 sits at the Dornier Museum in Friedrichshafen, a few kilometres from where it was designed. E3 — the one that actually hovered, the one Drury Wood flew backwards — is at the Deutsches Museum Flugwerft Schleissheim outside Munich. It is the only aircraft of its type anywhere, and it always will be.

Sources: Andrew Dow, Pegasus: The Heart of the Harrier (2009); Hirschel, Prem and Madelung, Aeronautical Research in Germany (2012); Paul A. Jackson, German Military Aviation 1956–1976; Fédération Aéronautique Internationale record files; Deutsches Museum; Dornier Museum Friedrichshafen.