It originated as the Avia BH-17 during the early 1920s, which was redesigned in response to feedback from an official evaluation of the aircraft as a fighter.

The origins of the BH-21 can be traced back to the Avia BH-17, a conventional biplane designed by Pavel Beneš and Miroslav Hajn during the early 1920s as a response to a Czechoslovak Defense Department requirement for a new fighter aircraft.

[citation needed] The Avia BH-21 was a capable fighter aircraft that possessed favourable performances across multiple categories.

[2] The aircraft had a rectangular fuselage that was entirely covered with plywood, which permitted the omission of brace wires.

[2] The pilot's cockpit was located directly behind the cell; it was relatively large and comfortable, featuring an adjustable seat and made provision for a dorsal parachute.

[4] The wings were braced on each side of the fuselage by N-shaped struts of streamlined steel tubing and two pairs of double cables.

The brace wires were attached to the lower part of the fuselage and the upper wing, achieving a favourable distribution of stresses.

[4] These ailerons were unbalanced, had a steel tube framework and a fabric covering; they were actuated via a system of tubular rods and levers.

[2] Typically, the powerplant was a single Hispano-Suiza 8Fb V-8 water-cooled piston engine, which was capable of producing up to 300 HP at 1,BOO rpm.

Cooling was accomplished by a honeycomb-style radiator, produced inhouse by Avia, that was located underneath the fuselage; it can be raised and lowered as required.

[7] During 1925, quantity production of the BH-21 commenced, a total of one hundred and thirty-nine aircraft were produced for the Czechoslovak Air Force.

In June 1925, it was successful in trials staged by the Belgian Air Force; this led to one aircraft being built for Belgium by Avia while another thirty-nine were produced under license by the Belgian company Société Anonyme Belge de Constructions Aéronautiques SABCA and five by the Société d'Etudes Général d'Aviation SEGA.

[citation needed] Data from Les Ailes, May 1925,[9] National Advisory Committee for Aeronautics[10]General characteristics Performance Armament