The success of the Dolphin, a military amphibian, helped Douglas survive the Great Depression. It began as the twin-engine Sinbad, intended to be a luxury air yacht. It was a high-wing monoplane, with its engines mounted above the wing. Its six to eight passengers looked out picture windows, and their baggage was stored in a 30-cubic-foot area. Sinbad had no market during the luxury-deprived era. However, 59 of the next version, the Dolphin, were built between 1931 and 1934. The Dolphin retracted its landing gear for water landings and was adapted to meet customer requirements, both military and civilian. It evolved into 17 variants. among the first purchasers were the Wilmington - Calantina Airline and Standard Oil of New Jersey. The U.S. Army, Navy and Coast Guard bought the Dolphin in quantities. Some military Dolphin remained in service until World War II. The wealthy Vanderbilt family bought two. One, called Rover, was sold to William Boeing, who took delivery of his Dolphin in 1934, just before he left the company he founded. Still flying in 1977 it was the last known survivor of the Dolphin series.