Douglas Dolphin

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.
Operational history.
The first two were purchased by Wilmington-Catalina Airlines to fly passengers between Los Angeles and Santa Catalina Island, becoming the first successful Douglas airliners. Subsequent examples were ordered by the United States Navy and U.S. Coast Guard for use as transports and search and rescue craft. The U.S. Army Air Corps ordered several under the designations C-21, C-26, and C-29. Many were eventually ordered for their original purpose as luxury transports. Owners included William Boeing, the founder of the Boeing Company, and Philip K. Wrigley, the son of the founder of the Wm. Wrigley Jr. Company. William K. Vanderbilt bought two with custom interiors for use from the Vanderbilt yacht Alva as flying tenders.
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.
Variants.
"Data from:McDonnell Douglas Aircraft since 1920 Vol.1"