The Douglas Dolphin was an American amphibious flying boat. While only 58 were built, they served a wide variety of roles including private air yacht, airliner, military transport, and search and rescue.
Design and development
The Dolphin originated in 1930 as the "Sinbad," a pure flying boat without wheels. The Sinbad was intended as a luxurious flying yacht. Undaunted by the lack of demand, Douglas improved the Sinbad in 1931 so that it was amphibious, and could land on water or land. The improved aircraft was named "Dolphin", however this did not represent the end of development, as many detail improvements were made, including an increase in the length of over a foot and changes made to the empennage, engine nacelles and wings. The Great Depression had curtailed demand for such extravagance as a "flying yacht", but Douglas managed to interest the United States Coast Guard who not only bought the Sinbad, but 12 Dolphins.
A U.S. Coast Guard RD-1.
Douglas OA-4A with tricycle landing gear at Langley
One was procured by the U.S. Navy as a transport for President Franklin D. Roosevelt. Although never used by Roosevelt, this was the first aircraft procured to provide transportation for the President of the United States.
In 1933, landing in heavy seas, the USCG RD-4 undertook some rescues of merchant sailors at sea, feats that made spectacular news reports, enthralling the American public.
A RD-4 in World War II
Data from:McDonnell Douglas Aircraft since 1920 Vol.1
The original prototype built as a flying boat, intended to be a luxurious flying yacht, first flown in July 1930. No orders were received for the Sinbad which was eventually bought by the U.S. Coast Guard.
Dolphin Model 1
The initial two Dolphins built for the Wilmington-Catalina Airline Ltd. as six-seat airliners.
Dolphin Model 1 Special
The Model 1s redesignated after modification to seat eight passengers.
Ten aircraft for the U.S. Coast Guard, powered by 2x 420 hp (313 kW) Pratt & Whitney Wasp C1 engines. One, V-126, was destroyed on the morning of August 5th, 1941, when it likely struck a rock pinnacle on the southeastmost of the Farallon Islands near San Francisco, causing the aircraft to burst into flames. All 3 crewmen aboard were killed. 
C-21 aircraft redesignated.
C-26 aircraft redesignated.
Y1C-26A aircraft redesignated.
C-26B aircraft redesignated, one of which was fitted with an experimental fixed tricycle undercarriage.
Four OA-4A and one OA-4B aircraft modernized in 1936.
Eight aircraft for the USAAS, similar to the Navy's RD-1, powered by 2x 350 hp (261 kW) Wright R-975-3 engines.
Two aircraft for the USAAS with increased dimensions, fin area and fuel capacity (from180 US gal (681 l) to 240 US gal (908 l)). Powered by 2x 300 hp (224 kW) Pratt & Whitney R-985-1 engines.
Eight aircraft for the USAAS differing from the Y1C-26 only in minor details.