After the first World War and in the early twenties, the Belgian Aviation Militaire was equipped with a mix of aircraft originating from countries as diverse as France, Great Britain and even Germany (“butin de guerre” Fokker D.VII's) creating what we now would call "a logistical nightmare". As new and more sophisticated aircraft demanded ever increasing technical support, the need to standardise on one single fighter type became more and more urgent. (this would only be concretized with the introduction of the Fairey Firefly in 1931). The most important fighter in service at that time, the Nieuport NiD 29 C.1 was not considered a major improvement over the WWI fighters and due to its inability to perform aerobatics caused by its fragility it was a design not particularly liked by its pilots.

Meanwhile in Czechoslovakia the Avia aeronautical company founded in 1919 started building simple and strong designs, which caught the attention of the aeronautical world by winning many a trophy in the "Concours" organised all over Europe. In January 1925 the Avia BH-21 successfully made its first flight near Prague and by 1926 the Belgian Government started showing interest in the new design. For evaluation purposes the Defence Ministry bought three different types of fighters: the Dewoitine D19 C.1, the Avia BH-21 and a Jupiter (420 hp) powered Dewoitine D.9. Unlike today, no competing "fly-off" was organised, but instead the aircraft were delivered to the operational squadrons for individual testing and evaluation.

As the Avia was powered by the same Hispano-Suiza 300hp engine which equipped the Nieuport NID 29 (large stocks of this engine being available as spares in the Aviation Militaire) and as it also had a remarkable aerobatics capability, the Avia was favoured by the Belgian authorities. Very soon the decision was taken to order some 44 BH-21's. Five machines were to be constructed by Avia while for the remaining 39 aircraft a contract was signed for a license production by Sabca in Belgium.

The first Avia fighters were delivered in 1927 to the 1st Groupe of the 2nd Regiment (I/2 Aé) based at Schaffen (the first Belgian built BH-21 arrived on 14 September 1927) and served together with the Nieuport NiD 29 C.1 at Schaffen and Nivelles.  Although they were meant to be replaced by the Avia BH-21, for an unknown reason the Nieuports soldiered on alongside the Czech fighters in Belgian service until the early thirties. With the arrival in July 1931 of the first Fairey Firefly both types were gradually replaced by this much more modern and capable fighter. Some Avia's were transferred to the Pilot School at Wevelgem were they were finally retired in 1934.

So the ultimate destiny of this powerful and much liked little machine from Czechoslovakia was to serve as an interim fighter to fill the gap between the obsolete Nieuport NiD 29 and the next generation Fairey Firefly and to keep Sabca's licence production lines up and running.