On 21 October 1929, a flying boat took off from Lake Constance with 169 people aboard. No aircraft had ever carried close to that many. The record would stand for about twenty years.

The Dornier Do X had twelve engines mounted above its wing in six back-to-back pairs, three passenger decks, a smoking room with a wet bar, and a flight engineer who worked twelve throttles from a separate control room and took his orders by engine-order telegraph, exactly as on a ship. Because that is what Claude Dornier had built: an ocean liner that happened to fly.

Barely.

Built in Switzerland on purpose

The Treaty of Versailles capped what German factories were allowed to build. Dornier’s answer was to build it somewhere else. The works went up at Altenrhein, on the Swiss shore of Lake Constance, a few miles from Germany and comfortably outside the treaty’s reach. Construction began in December 1927 and took something in the region of 240,000 working hours.

What emerged was, by a wide margin, the largest and heaviest aircraft in the world. It had no wing floats; instead it used sponsons — stub wings low on the hull that steadied it on the water and doubled as boarding platforms. The lower deck held the fuel and nine watertight compartments, only seven of which were needed to keep it afloat.

The aviation press was, briefly, lost for words.

169 people, no passenger certificate

The October 1929 flight was a demonstration, and a slightly reckless one. The aircraft held no passenger certification at all. The people aboard were mostly Dornier workers and their families, plus journalists and crew, and depending on which account you read there were either nine stowaways in the total or none at all. The aircraft climbed to around 200 metres, flew a circuit over Bregenz, Lindau and Friedrichshafen, and came back down after roughly fifty minutes.

That was written four days after the event. Ninety-seven years later the exact breakdown of those 169 people is still disputed. The total, at least, everyone agrees on.

The problem with twelve engines

The original powerplant was twelve Siemens-built Bristol Jupiter radials, each around 525 horsepower. On paper that is 6,000 hp. In practice they overheated at the sustained high revolutions the aircraft demanded, and the Do X could barely drag itself to 425 metres.

Read that again. An aircraft with a quoted service ceiling of 3,200 metres could not reliably get above about four hundred. It was, functionally, a machine that flew in ground effect and slightly above it. After 103 flights the Jupiters came out and twelve water-cooled Curtiss Conqueror V-12s went in, which bought it roughly another hundred metres of altitude — enough, just, to attempt an ocean.

Managing all this fell to a flight engineer in his own compartment on the upper deck, plus a team of mechanics who crouched in a crawlway barely over a metre high inside the wing, each nursing two engines.

Ten months to New York

In November 1930 the Do X set off for America. It went the long way, because it had to: Netherlands, England, France, Spain, Portugal. At Lisbon a fire destroyed the covering of the left wing — contemporary reporting in Flight blamed an electrical fault — and the aircraft sat there for around six weeks being repaired.

Then down the West African coast, out to the Cape Verde islands, across the Atlantic at its narrowest point to Natal in Brazil, up through Rio and the Caribbean, and finally into New York on 27 August 1931. Elapsed time from Friedrichshafen: very nearly ten months.

It then spent nine months in New York having its engines overhauled while sightseers came to look at it, and returned to Berlin in May 1932 to a crowd reported at 200,000 people.

What happened to them

Three were built. The German aircraft was withdrawn after a heavy landing on a reservoir near Passau in 1933 tore the tail off — an accident that was quietly not publicised — and ended up as the centrepiece of the German aviation collection in Berlin, where it was wrecked in a bombing raid in November 1943. The two Italian machines, powered by Fiat engines, were mothballed at Marina di Pisa in 1935 and broken up for scrap two years later.

The verdict that matters was reached almost immediately. In that same October 1929 editorial, having done the arithmetic on payload against power reserve, Flight concluded that as a commercial proposition the Do X might not be quite so favourably placed as it first appeared. It took Dornier another three years to agree.

Period footage, from the Dornier Museum’s own archive:

And a 1930 film of a flight aboard it, which gives a far better sense of the scale of the thing than any specification:

More archive film of the aircraft in service:

Sources: Flight, 25 October 1929 and 21 February 1930; Der Spiegel einestages, 2009; Dornier Museum Friedrichshafen; Bundesarchiv.