The M.21 was powered by an uncowled radial engine, either the 96 hp (72 kW) Siemens Sh 11 (M.21a) or the 125 hp (93 kW) Sh 12 (M.21b) driving a two-bladed propeller.
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The BFW M.21 was a single-engined biplane trainer designed by Willy Messerschmitt for the German Ministry of Transport.
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During 1927, following the success of his M 18 airliner tried to gain funding from the Baverian government; because the latter were already subsiding BFW (Bayerische Flugzeug-Werke) they pressed for an arrangement whereby Willy Messerschmitt joined BFW on the understanding the company produced only his designs.
These were distinct from the Romanian modified version known as the ICAR Universal.
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Other went to individual owners, with some familiar names amongst them like Ernst Udet (who made well publicised flights to Africa and to Greenland, the latter with Leni Riefenstahl as a passenger) and Rudolph Hess.
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In 1933, Erwin Aichele and his wife flew trouble-free for 13,000 km (8,000 mi) around the Mediterranean.