04/30/2010. In July 1930 Donald Douglas, for whom the sea always retained its fascination, combined his vocation and his avocation by building the first flying-boat of his own design, the Sinbad. Prototype of the Dolphin series, the twin-engined Sinbad was conceived in the economic boom of the late 1920s as a luxury 'air yacht'. Unfortunately, in the civil market, the aircraft was a victim of the Great Depression and Douglas was saved from incurring a telling financial blow only through the fortuitous success of the military versions of the Dolphin.
The Sinbad (c/n 703, NX145W) was a high-wing cantilever monoplane of wood and metal construction, with all-metal hull and wood-covered cantilever wing. Two pilots sat in an enclosed cabin, located just forward of the wing, and for eight passengers in a cabin within the hull and beneath the wing. A pair of 300 hp Wright J-5C Whirlwind radials were mounted above and forward of the wing on multiple struts (initially faired over to reduce drag), and of tube-braced tail planes.
While the Sinbad was being tested, Douglas was planning the production of the Dolphin, an amphibian version based on the modified Sinbad, fitted with retractable undercarriage, and intended for military and civil markets. While only 58 Dolphins were built between 1931 and 1934, this total included not less than 17 variants and models as the Dolphins were either custom-built for each civil customer or produced in small batches for military customers. Furthermore, progressive improvements, modifications and/or engine changes were introduced on the line.
Ordered as Y1C-21s, c/n 1075 to 1082, for the USAAC under Contract AC-4460 of 10 August, 1931, these eight aircraft (s/n 32-279 to 32-286) were powered by two 350 hp Wright R-975-3 engines. Initially, the USAAC intended to assign one or two of them to each Bombardment Group with the idea that they would fly along with the bombers during overwater flights to act as navigators and rescue aircraft. However, mainly due to their relatively low speed, which prevented them from keeping up with the bombers, the idea was soon dropped.
Later, they were redesignated plain C-21s, in the transport category, and then OA-3s, in the observation amphibian category. So designated, they were used as transport and rescue aircraft, and an OA-3 assigned to the 7th Bombardment Group at Hamilton Field, California, pioneered for the USAAC the technique of air-sea rescue operations. Some of these aircraft, temporarily designated FP-1s, were also loaned to the Treasury Department for use on border patrols during Prohibition.
Eventually, the OA-3s, with the exception of 32-281 which was destroyed on November 23, 1938, in a taxiing accident at West Point, were assigned to the Panama Canal Zone, the Territory of Hawaii, and the Philippine Islands. The last was withdrawn in March 1940.