In December 1967, at the Dornier flight-test centre at Friedrichshafen, a strange aircraft lifted off the apron without using a runway. It had two main jet engines in conventional underwing pods and ten — yes, ten — additional pure lift engines clustered in pods at the wingtips. It rose vertically to about 30 metres, hovered, then transitioned to forward flight and accelerated away. It was the Dornier Do 31, and as of today, fifty-nine years later, it is still the only jet-powered VTOL transport aircraft to have ever flown.

Germany built three of them. None entered service. The programme was cancelled in 1970. And nobody, in the half-century since, has built anything like it.

Why Try?

The Do 31 was born from a 1960s NATO requirement (NBMR-4) for a tactical transport that could operate without runways. The reasoning was Cold War-pragmatic: Warsaw Pact forces were expected to target every NATO airfield in West Germany with nuclear or conventional strikes in the first hours of a war. If transport aircraft could not depend on runways, they had to do without them.

Dornier, with experience from the earlier Do 29 conventional STOL experiments, proposed a fundamentally radical answer — vertical takeoff with separate lift engines, supplemented by vectored-thrust main engines, then horizontal cruise on the main engines alone with the lift engines shut down.

The Engineering Problem

Pure-jet vertical lift is extraordinarily wasteful. The lift engines on the Do 31 burned fuel at an enormous rate, did nothing useful in cruise, and added significant weight and drag throughout the flight. The aircraft’s lift-to-weight ratio at vertical takeoff exceeded 1.05 only with full power on all twelve engines simultaneously, with negligible reserve.

Transitioning from hover to cruise was, by all accounts, an exacting piece of flying. The pilot had to manage thrust on twelve engines, vector the two Pegasus mains forward as horizontal speed built up, and shut down the lift jets in sequence as the wing began producing aerodynamic lift. Any phase-mismatch produced either a stall or a thrust shortfall.

Why It Was Cancelled

The Do 31 worked. It flew vertically. It transitioned cleanly. It set FAI records for VTOL speed and altitude. By every engineering measure it was a success.

Operationally, however, the case fell apart. The lift engines reduced payload by roughly 40 percent compared to a conventional STOL transport of the same gross weight. Fuel burn in vertical flight was unsustainable. And — most importantly — the helicopter caught up. By 1970, the CH-53 Sea Stallion and CH-47 Chinook offered comparable payloads, far better fuel economy, true point-to-point flexibility, and operational maturity. The Do 31 could not compete with the rotary-wing solution.

NATO quietly withdrew the requirement. Dornier had no follow-on customer. The programme ended in April 1970.

“The success of an aircraft depends on the fact that it is the result of clear thinking — and a deep understanding of the physical laws which govern the air.”

Claude Dornier (1884-1969) — Founder of Dornier-Werke, whose final company project was the Do 31

An Idea Without a Successor

No jet VTOL transport has flown since. The military VTOL category is owned by the V-22 Osprey tiltrotor, and the commercial VTOL space is now dominated by eVTOL electric concepts. The pure-jet lift architecture of the Do 31 has been quietly retired from the engineering vocabulary.

One surviving prototype sits today at the Deutsches Museum Flugwerft at Oberschleißheim, outside Munich. It is large, surprisingly graceful, and looks oddly modern from certain angles. It is also the last aircraft of its kind that the world will ever see.

The Dornier Do 31 — the world’s first and only jet VTOL transport.

Sources: Deutsches Museum, Dornier company archives, Flight International archives.