In late 1936 Grumman began the design of a small six/seven-seat commercial amphibian flying-boat, identified as the G-21. First flown in June 1937, it was of high-wing cantilever monoplane configuration. Of all-metal basic construction, part light alloy-/part fabric-covered, the wing included split trailing-edge flaps. The all-metal two-step hull incorporated retractable tailwheel-type landing gear, all units retracting into recesses formed in the hull. Stabilising floats were strut-mounted and wire-braced beneath the outer wing panels. The powerplant comprised two wing-mounted Pratt & Whitney R-985 Wasp Junior engines with variable-pitch propellers. The pilot's compartment, forward of the wing, seated two side-by-side; the aft cabin had standard seating for four passengers, a lavatory and room for baggage.

These small flying-boats were of immediate interest to the US armed services, the US Army losing little time by ordering 31 under the designation OA-9, and these began to enter service in an observation/communications capacity during 1938. One of these was still in service in 1948, when it was redesignated ZA-9. Five Geese were procured for Lend-Lease to the Royal Navy but were reposessed for USAAF use. These became A-9s in 1948. Three additional G-21As were purchased from private owners in 1942, and these became designated OA-13A. Two US Navy JRF-5s, transferred in 1945, became designated OA-13B.