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Douglas Sinbad / Dolphin / C-21 / C-26 / C-29 / OA / RD
Completed in July 1930 as the non-amphibious Sinbad under civil registration NX145Y, Douglas’s first in-house flying boat design was initiall concieved as a “flying yacth” to be offered on the civilian market. The Sinbad appeared as a monoplane having an all-metal hull of semi-monocoque construction and a two-spar cantilever wing covered in plywood that featured slotted, Handley Page type ailerons. In original configuration, tne 300-hp Wright J-5C Whirlwind engines were mounted directly above the wing and encased in nacelles that blended-in with its upper surface.
 Several men examine Douglas Aircraft Co.'s new amphibian plane "Sinbad" at Clover Field (Santa
Monica Airport). She became the prototype for the Dolphin series. Photo taken on July, 17, 1930,
around the time of its first flight.
After flight testing revealed the need to raise the thrust line, the engins were moved above the wing on struts, along with an auxiliary airfoil mouned between the conical engine nacelles to add structural support and lift. When no civilian buyers surfaced, the Sinbad was sold to the Coast Guard in March 1931 for $31,500, where it operated for a period of time as call-sign “24 G” without a military designation but later simply listed as the “RD” with no numeric suffix.
 Sinbad after Modifications
The improved Dolphin, equipped with amphibious landing gear, a modified hull, and 400 hp Pratt & Whitney R-1340 engines, emerged in early 1931 and two, known as the Dolphin I Speccial, were sold n June the same year to the Wilmington-Catalina Airline where they completed close to 40,000 crossings without incident. Military orders followed for 350 hp R-975 Whirlwind powered examples for the US Army Air Corp and another to the Navy as the XRD-1, both delivered in late 1931. Twenty-three more Dolphin variats were procured for the navy and the Coast Guard between 1932 and 1934; three RS-2s in early 1933, two to the Navy and one to the Coast Guard, powered by 450 hp Pratt & Whitney R-1340 engines; six very similar RD-3s in mid-1933 to the Navy; and ten RD-4s in late 1934 to the Coast Guard, powered by 450 hp R-1340-96 engines. One of the Navy RD-2s was specially outfitted for President Franklin D. Rooservelt but there is no record of it being used for such purpose. Navy RDs were assigned to utility squadrons and used primarily as transports, whereas Coast Guard versions saw extensive service in the search and rescue role as flying lifeboats. Two RD-3s were subsequently assigned to the Marine Corps to be used as utility transports. One aircraft reportedly manufactured as an RD-2 was used as a government transport by the Secretary of the Treasury until 1937. After the US entered World War II, RD-4s remaining in service with the Coast Gusrd were employed briefly for coastal atrol duties.
The authorities are in conflict as to the exact number of Dolphins accepted by the US Army Air Corp, however, it appears at least sixteen were delivered between 1932 and 1933. Two in 1932, powered by 300 hp Pratt & Whitney R-985 engines, eight in 1932 powered by 350 hp R-985-5 engines, as the C-26A; four in 1932-33, powered by 350 hp R-985-9 engines, as the C-26B, and two with enlarged cabins in 1933, powered by 575 hp R-1349-16 engines, as the C-29.
 RD-2
In 1936-37 four OA-4As and one OA-4B were refitted with stainless steel wings, thn another OA-4B became the OA-4C when modified with experimental tricycle landing gear.
Civilian Dolphins, of which sixteen (including the two Dolphin I Specials) are estiated to have been built between 1933 and 1934, came in many variations ae decreaed by the preferences of individual customers. One registered as NC14286 was completed as a personal transport for William E. Boeing. French industrialist Armand Esders purchased a Dolphin powered by two 550 hp R-1340-51 engines, Standard Oil Company acquired two with 450 hp R-1340-56 engines to be used in overseas operations, and two more were built for the Vanderbilt family. In 1934 Pan American Airways bought two Dolphins that were operated by China National Airways Corp subsidary. After World War II began, one Dophin I Special and one from Standard Oil ended up flying in Australia with the RAAF.
 NC14208 custom-built for the Vanderbilt family
Total produced (all versions) 59
Sinbad
Engines: 2 x Wright J-5C Whirlwind, 300 hp
RD-4
Engines: 2 x Pratt & Whitney R-1340-96 Wasp, 450 hp
Props: 2 blade, ground adjustable, metal
Wingspan: 60 ft 10 in / 18.29 m
Wing area: 592 sq.ft / 55.0 sq.m
Length: 45 ft 3 in / 13.74 m
Height: 14 ft / 4.27 m
Empty weight: 7000.9 lb / 3175.0 kg
Gross weight: 9737 lb / 4323.0 kg
Max speed: 147 mph / 136 kts / 251 km/h
Cruise speed: 110 mph / 117 kts / 217 km/h
Ceiling: 14,900 lb
Range: 660 mi
Seats: 8
C-21
Engine: 2 x Wright R-975-3, 261kW
Max take-off weight: 3893 kg / 8583 lb
Loaded weight: 2659 kg / 5862 lb
Wingspan: 18.29 m / 60 ft 0 in
Length: 13.36 m / 43 ft 10 in
Height: 4.29 m / 14 ft 1 in
Wing area: 52.21 sq.m / 561.98 sq ft
Max. speed: 225 km/h / 140 mph
Cruise speed: 192 km/h / 119 mph
Ceiling: 4330 m / 14200 ft
Range: 885 km / 550 miles
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