<p>In 1930 Douglas introduced a new twin-engined commercial amphibian flying-boat which it had named Dolphin. Powered by two radial engines, strut-mounted above the high-set cantilever monoplane wing, it provided accommodation for a pilot, co-pilot and six passengers. An unusual feature was the use of an aerofoil section structure to brace together the two engines. An attractive looking boat, it soon aroused the interest of the US armed forces, which were seeking transport amphibians to supplement the Loening observation amphibians then in service with both the US Army and US Navy.
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<p>First to acquire the Dolphin, however, was the US Coast Guard, which ordered three of the standard commercial aircraft in 1931 under the designation RD, the first of these being delivered on 9 March 1931. The next service to procure these aircraft was the US Army Air Corps, which in 1932 ordered eight Y1C-21s, with 350-hp (261-kW) Wright R-975-3 Whirlwind engines and accommodation for a crew of seven, plus two eight-seat Y1C-26s with 300-hp (224-kW) Pratt & Whitney R-985-1 Wasp Juniors. Subsequent orders covered eight Y1C-26As and six C-26Bs, powered by R-985-5 and R-985-9 engines, respectively, both types being rated at 350 hp (261 kW). In 1934, two C-26Bs were converted to accommodate a crew of two and seven passengers, being redesignated C-29. In that same year the US Army changed the role of the aircraft, using them as observation amphibians with a crew of four; this brought designation changes with Y1C-21, Y1C-26, Y1C-26A and C-26B versions becoming, respectively, OA-3, OA-4, OA-4A and OA-4B. One OA-4 was provided experimentally with non-retractable tricycle-type landing gear: this enabled the USAAC to evaluate the potential of this landing gear configuration, leading to its specification for use initially on the Douglas C-54 transport.
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<p>US Navy use of the Dolphin began with the procurement of one aircraft, comparable with the US Coast Guard RD and US Army Y1C-21, which was delivered in December 1931 and designated XRD-1. Three RD-2s followed, these differing by having strengthened mounting struts for the overwing engines, which were higher-powered Pratt & Whitney R-1340-96 Wasps, and six additional RD-3s of generally similar configuration were next procured. From these aircraft the US Navy subsequently allocated one RD-2 and two RD-3s for service with the US Marine Corps. Final procurement covered the production of 10 RD-4s for the US Coast Guard, these being virtually identical to the US Navy's RD-3s except for minor changes in detail and equipment.
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<p>In American service the US Army's Dolphins were used primarily for transport or patrol duties, and those of the US Navy as personnel transports, being of great value for communications between ship and shore. The US Coast Guard, however, used its small fleet in a search and rescue role. When the USA became involved in the war, all services used these aircraft extensively in the early years for transport and rescue, as well as for security patrols along the nation's coastline.
<item><hi format=bold>Powerplant (RD-4):</hi> two 450-hp (336-kW) Pratt & Whitney R-1340-96 Wasp radial piston engines
<item><hi format=bold>Performance:</hi> maximum speed 156 mph (251 km/h); cruising speed 135 mph (217 km/h); service ceiling 17,000 ft (5,180 m); range 720 miles (1,159 km)
<item><hi format=bold>Weights:</hi> empty 7,000 lb (3,175 kg); maximum take-off 9,530 lb (4,323 kg)
<item><hi format=bold>Dimensions:</hi> span 60 ft 0 in (18.29 m); length 45 ft 1 in (13.74 m); height 14 ft 0 in (4.27 m); wing area 592 sq ft (55.00 m<su>2</su>)
