The Dutch shipbuilding company NV Koninklijke Maatschappij De Schelde of Vlissingen entered the aviation business in 1935, when it recruited most of the technical staff, including Chief Designer T. E. Slot, of Pander & Son, which had failed in 1934. Early designs built by De Schelde were the De Schelde Scheldemusch, a lightweight single-seater pusher biplane powered by a Praga B engine, and the Scheldemeeuw, a flying boat version of the Scheldemusch.Beeling Air International December 2015, pp. 73–74. In 1938, Slot started design work on two more advanced types which also followed Slot's preferred pusher configuration, the De Schelde S.20, a light cabin monoplane, and the S.21, a single-seat fighter, with construction of prototypes for both types beginning in early 1939.Beeling Air International December 2015, p. 74.
