In June 1935 the prototype Model C-34 made its first flight, piloted by George Harte. Powered by a 108kW Warner Super Scarab, this was a four-seater which demonstrated its aerodynamic efficiency by achieving a maximum speed of 261km/h, compared with 238km/h for the much higher-powered DC-6B. Some 42 were built, two being impressed for USAAF under the designation UC-77B.

The Model C-37, introduced during 1937, brought in minor improvements to the furnishing of the cabin, which was widened by 12.7cm, and was fitted with electrically operated wing trailing-edge flaps. Some 46 C-37s were built, and one impressed in 1942 was designated UC-77C.

The 1938 Model C-38 again featured minor improvements, with some of the previously optional equipment fitted as standard; airframe changes included the provision of wider track landing gear and a large under-fuselage flap which served as an airbrake to reduce landing speed. Production of the C-38 amounted to 16 aircraft.