The Dornier Do X was the largest, heaviest, and most powerful flying boat in the world when it was produced by the Dornier company of Germany in 1929. First conceived by Claude Dornier in 1924, planning started in late 1925 and after over 240,000 work-hours it was completed in June 1929.

During the years between the two World Wars, only the Soviet Tupolev ANT-20 Maksim Gorki landplane of a few years later was physically larger, but at 53 metric tonnes maximum takeoff weight it was not as heavy as the Do X's 56 tonnes.

The Do X was financed by the German Transport Ministry and in order to circumvent conditions of the Treaty of Versailles, which forbade any aircraft exceeding set speed and range limits to be built by Germany after World War I, a specially designed plant was built at Altenrhein, on the Swiss side of Lake Constance.

The type was popular with the public, but a lack of commercial interest and a number of non-fatal accidents prevented more than three examples from being built.