The Curtiss CS (or Model 31) was a reconnaissance and torpedo bomber aircraft used by the United States Navy during the 1920s. It was a large single-engine biplane with single-bay unstaggered wings, the design conventional in all respects other than the fact that the lower wing was of greater span than the upper. The CS was built to allow its undercarriage to be quickly and easily interchangeable between wheeled, tailskid undercarriage, and twin pontoons for operation from water.