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The Douglas C-26/ OA-4 Dolphin was an improved version of the C-21/ OA-3 Dolphin, with a number of improvements over the earlier aircraft.
The C-21/ OA-3 was a twin engined amphibian, with two engines mounted above the wings. It was based on the commercial Douglas Sinbad, but most were used by the US Army, which originally purchased them as bomber leaders, but then used them as transports and air-sea rescue aircraft.
The original Dolphin had auxiliary fins on the tail. On the 14th aircraft a new enlarged fin and rudder was introduced, which solved the aircraft's directional instability. As a result the auxiliary fins could be removed. The wing area was also increased, as was the length of the aircraft. Fuel capacity also went up.
The US Army Air Corps ordered a number of the redesigned Dolphins. They were originally designated as the C-26. When loaned to the US Treasury they became the FP-2, and eventually they were redesignated as the OA-4 (Observation, Amphibian).
Y1C-26
The first two modified Dolphins were ordered by the US Army Air Corps on 7 May 1932 and delivered in 1933. They had the new tail and no auxiliary fins, and were powered by two 300hp Pratt & Whitney R-985-1 radial engines. They later went to the Treasury as the FP-2, were redesignated as the OA-4, then were modernised in 1936 as the OA-4C.
C-26A
The C-26A was similar to the basic C-26. Eight were ordered on 10 June 1932 (Contract AC-5100) in two batches of four (as a result they had non-consecutive Douglas serial numbers but consecutive US Army numbers. They entered service as the Y1C-26, then became the C-26A and then the OA-4A.
C-26B
Four C-26Bs were ordered on the same day as the more powerful C-29 Dolphin. The C-26B was powered by two 400hp R-985-9 engines and they were delivered in 1933. One was later used to test a non-retractable tricycle undercarriage. They were later redesignated as the OA-4B, and one was updated to OA-4C standard in 1936.
FP-2
The designation FP-2 was given to the two Y1C-26s when they were loaned to the US Treasury for use on Prohibition patrols.
FP-2A
The FP-2A was the designation given to the C-26A when it was loaned to the Treasury.
OA-4
The designation OA-4 was given to the two Y1C-26s when they were first redesignated as Observation, Amphibious types. They were later modernised to become the OA-4C.
OA-4A
The OA-4A was the designation given to the eight C-26As when they were redesignated as amphibious observation aircraft. Four were later modified to become OA-4Cs.
OA-4B
The designation OA-4B was given to the four C-26Bs when the type was redesignated as observation, amphibians. One was later modernised to the OA-4C standard.
OA-4C
In 1936 two Y1C-26s/ OA-4s, four C-26A/ OA-4As and one C26B/ OA-4B were modernised by giving them stainless steel wings and 400hp R-985-9 engines. At the same time they were redesignated as the OA-4C.
Y1C-26A
Engine: Two Pratt & Whitney R-985-1
Power: 300hp each
Crew: 2
Span: 60ft
Length: 45ft 10in
Height: 14ft 1in
Empty weight: 5,8671lb
Loaded weight: 8,571lb
Max speed: 137mph at sea level
Climb Rate: 21.1min to 10,000ft
Service ceiling: 13,200ft