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The Douglas RD was the Navy version of the Douglas Dolphin twin-engined amphibious aircraft, and was produced in a number of variants for the Navy and Coast Guard. In army service it was known as the C-21/ OA-3 (see here for technical details) or the C-26/ OA-4.
XRD-1
The XRD-1 was the first version of the Dolphin to serve with the US Navy. It was powered by two 435hp Wright R-975E radial engines, and carried military instruments. It was accepted by the Navy on 1 August 1931, and was used as a US Navy staff transport until it was struck off on 12 July 1938.
RD
The sole RD was built as a civil Model 1 Special, powered by two 300hp Wright J-5C Whirlwing engines and with space for eight passengers. In August 1932 it was purchased by the US Coast Guard, and given two 435hp Wright R-965E engines. It served with the Coast Guard until November 1939.
RD-2
The designation RD-2 was given to a number of aircraft, all similar to the C-26/ OA-4, with a modified tail and no auxiliary tail fins.
RD-2 s/n 1122 was purchased by the Air Corps for the Coast Guard. It was similar to the Army's Y1C-26, but was powered by two 500hp Pratt & Whitney R-1340-10 engines.
RD-2 s/n 1138 was a five-place luxury transport used by President Roosevelt from 1933 until 1939. It was originally powered by two 410hp Pratt & Whitney R-1340-1 engines and later by two 500hp R-1340-10 engines.
RD-2 s/n 1139 and 1140 were less luxurious transport aircraft used by Navy and Marine officers. They were powered by two 450hp R-1340-96 engines, and were retired in March 1940.
RD-3
The RD-3 was a utility transport powered by two 450hp Pratt & Whitney engines (either R-1340-4s or R-1340-96s). They were delivered in 1934-35.
RD-4
The RD-4 was the final military version of the Dolphin. Ten were ordered, making it the most numerous sub-type of the Dolphin. It was powered by two 420hp Pratt & Whitney Wasp C1 engines, and could carry 252 gallons of fuel. The first entered service in November 1934, and the type was used for search and rescue duties with the Coast guard.
In December 1941 four RD-4s were still in use when the US entry into the war brought the Coast Guard under direct Navy control. They were used to fly security patrols along the US coast. The last of the RD-4s was decommissioned in June 1943.
XRD-1
Engine: Two Wright R-975E radial engines
Power: 435hp each
Crew: 2
Span: 60ft 0in
Length: 43ft 3in
Height: 14ft 1in
Empty weight: 6,127lb
Loaded weight: 8,347lb
Maximum take-off weight:
Max speed: 151mph at 5,000ft
Climb Rate: 860ft/ min
Service ceiling: 17,300ft
Range: 466 miles
RD-3
Engine: Two Pratt & Whitney R-1340-4s or R-1340-96s
Power: 450hp each
Crew: 2
Span: 60ft
Length: 45ft 3in
Height: 15ft 2in
Empty weight: 6,764lb
Loaded weight: 9,734lb
Max speed: 149mph
Climb Rate: 6.2min to 5,000ft
Service ceiling: 15,100ft
Range: 692 miles