by 2x Pratt & Whitney R-1340-1 engines and later by 2x Pratt & Whitney R-1340-10 engines, seating five though it was reportedly never used by the President. RD-3 A utility transport version of the RD-2, six of which were built for the U.S. Navy, powered by 2x Pratt & Whitney R-1340-4 or by 2x Pratt & Whitney R-1340-96 engines. RD-4 Ten aircraft for the U.S. Coast Guard, powered by 2x 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. OA-3 C-21 aircraft redesignated. OA-4 C-26 aircraft redesignated. OA-4A Y1C-26A aircraft redesignated. OA-4B C-26B aircraft redesignated, one of which was fitted with an experimental fixed tricycle undercarriage. OA-4C Four OA-4A and one OA-4B aircraft modernized in 1936. Y1C-21 Eight aircraft for the USAAS, similar to the Navy's RD-1, powered by 2x Wright R-975-3 engines. Y1C-26 Two aircraft for the USAAS with increased dimensions, fin area and fuel capacity (from to ). Powered by 2x Pratt & Whitney R-985-1 engines. Y1C-26A Eight aircraft for the USAAS differing from the Y1C-26 only in minor details. C-21 Y1C-21 aircraft redesignated. C-26 Y1C-26 aircraft redesignated. C-26A Y1C-26A aircraft redesignated. C-26B Four aircraft powered by 2x Pratt & Whitney R-985-9 engines. C-29 Two Dolphins, powered by 2x Pratt & Whitney R-1340-29 engines, were essentially similar to the Y1C-26As. Military operators Argentine Naval Aviation Royal Australian Air Force U.S. Army Air Corps U.S. Army Air Force United States Coast Guard U.S. Marine Corps United States Navy Specifications (RD-3 Dolphin) See also References Notes Bibliography Bowers, Peter M. "Douglas Dolphin." Airpower, Volume 12, Number 6, November 1982. Francillon, René J. McDonnell Douglas Aircraft since 1920, Volume 1. London: Putnam & Company Ltd., Second revised edition, 1988, 1979. . External links Dolphin Flying boats 1930s United States civil utility aircraft 1930s United States military transport aircraft Amphibious aircraft High-wing aircraft Aircraft first flown in 1930 Twin piston-engined tractor aircraft