Developed from the S-36, the Sikorsky S-38 was an amphibian of sesquiplane configuration. To the main upper wing were attached a twin-boom tail and a series of struts connecting to a small fuselage hull and half-span lower wings. The latter mounted stabilizing floats. The twin tractor engines were mounted below the main wing on struts, but nevertheless high above the water to avoid spray ingestion.

A single example (29-406) was acquired for evaluation at Wright Field, following which an order for 10 C-6As (30-397/30-406) was placed. These were used during the early 1930s in the US, Canal Zone and the Philippines for utility transport and, on occasion, target towing.