The Cox-Klemin XS was a 1920s American experimental scout biplane, the first aircraft to be both launched and recovered from a submarine.

Based on a design by the US Bureau of Aeronautics for a simple single-seat scout seaplane that could be disassembled and assembled quickly.

As part of a series of studies conducted by the United States Navy after World War I into the possibility of submarine borne observation and scouting aircraft, the submarine S-1 became the experimental platform for this project late in 1923.

After surfacing the aircraft could be rolled out and assembled, it was then launched ballasting the sub until the deck was awash.

The first full cycle of surfacing, assembly, launching, retrieving, disassembly, and submergence took place on 28 July 1926, on the Thames River at New London using the XS-2.