Designed to achieve high speeds from modest engine power, it set seven class records in the early 1960s.

Between 1939 and 1975 the amateur aircraft designer and builder René Leduc (not the ramjet designer of the same name)[citation needed] completed and flew five different lightplanes, two of which set records in their class.

It was powered by a 101 kW (135 hp) SNECMA-Régnier 4L-00 four cylinder inverted air-cooled inline engine driving a two blade propeller.

A low, smoothly streamlined canopy covered the single seat cockpit and merged aft into a raised rear fuselage.

[4] Leduc built the RL.21 over a period of six years with assistance from Sud Aviation and from the Nantes Technical School.