In opposition to the 'low tech' policy of Fokker, the De Schelde firm decided to enter the market of the fighters in 1938 with the S.21, a sophisticated twin tail booms design built entirely of metal, spanning 8.5 m, and incorporating a number of innovatory features. It would have been powered by a 1,050 hp. DB 600 Ga, 12-cylinder inverted-Vee German engine driving a three bladed VDM, controllable-pitch, pusher propeller.
The armament would have consisted of one 7.9-mm MG 17 machine gun rear-firing through the propeller hub, for deterrent purposes, four 7.9-mm FN/Browning forward-firing machine guns and one flexibly mounted 20-mm Solothurn Tankbüsche S18-350 anti-tank cannon. During low-level attack, an automatic stabilising system controlling the ailerons and elevator, the pilot just had to operate the rudder to aim the cannon, which could be set in two positions: horizontal and 25-degrees downwards. The pilot used a retractable periscope to aim the MG 17. The propeller could be detached using explosive bolts when the pilot had to bail out.
The S.21 was expected to have excellent manoeuvrability and flight stability, due to the use of fixed wing slots and the position of the engine, installed over the CG. It was decided to change the engine to a 1,360 hp. DB 601 Aa, with turbocharger, in March 1940, to improve performance at high altitude. The prototype c/n 58 was captured by the Wehrmacht in May. The production version, with DB 601 Aa, would have had an estimated maximum speed of 590 Km/h and 10,000 m ceiling.