The Boeing 314 Clipper Flying Boat, 1938.

image002When the Boeing 314 flying boat made its appearance, it was the largest civil aircraft in service. 
The Yankee Clipper project dates back to 1935, with the start of a series of negotiations between Pan American World Airways and Boeing for the production of a flying-boat capable of guaranteeing transatlantic passenger flights with a high degree of safety, comfort and speed.
On July 21, 1936, Pan American signed a contract for six Model 314s, the first of which made its initial sea run on Puget Sound on May 31, 1938, and made its inaugural flight on June 7, 1938.
It outstripped all rivals in size, with twice the size of the Sikorsky S-42 and outweighed the Martin M-130 China Clipper by 15 tons. The 14-cylinder double-row Wright Cyclones were the first to use 100-octane fuel.
The Boeing 314 weighed 40 tons and the first block ordered cost $550,000 per aircraft.

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It had a central hull and adopted the wing and engine assembly of the experimental Boeing XB-15 heavy bomber.
In the place of the traditional floating stabilizers at the wingtips, sponsons (flotation device)  mounted on the sides of the hull were used.
The sponsons were based on the formula developed by the German engineer Claude Dornier and incorporated into such aircraft as the Dornier Do X and Dornier Do 18.
The sponsons (flotation device) also contained fuel tanks, the capacity of which (together with those situated in the wings) totaled almost 3,525 gallons (16,000 liters).
Read more via Boeing 314 Clipper.