The Martin S was a two-seat observation seaplane produced in the United States in 1915. Designed along the same general lines as the preceding Model T, it was a largely conventional two-bay biplane with unstaggered wings of equal span. The fuselage was not directly attached to the lower wings, but was carried on struts in the interplane gap. The undercarriage consisted of a single large pontoon below the fuselage and outrigger floats near the wingtips. The Model S was 23-year-old Donald Douglas's first and only design for the Martin company, and it set three world altitude records and a flight duration record that stood for three years.