Avia BH-21 - Czechoslovak users

The Avia BH-21 was built first in 1925 and was a single-seat biplane fighter that served with the Czechoslovak Army Air Force in the interwar period. It wasn't just a competent fighter but also a good racing airplane too.

It was designed by the same team composed by the engineers Pavel Benes and Miroslav Hajn who designed the BH-17, it's most direct predecessor.
The BH-21 and the BH-17 share some design features, in fact it could be said that the final BH-21 version was a heavily revised and modified BH-17. It's most welcomed feature was the improvement of the pilot's field of view. It was powered by a 300hp Hispano-Suiza 8fb which were built by Skoda under license and was armed with two 0.303in Vickers machine guns placed in the frontern upper part of the fuselage. It was put into production in 1925 and in one year 182 airplanes were produced, 137 of them were assigned to the Czechoslovak Army Air Force and the rest were either sold to Belgium or produced under license by the Belgian SABCA.
Two experimental variants were also built. The BH-21J, which was a regular BH-21 powered by a Bristol Jupiter engine which served as the predecessor for the BH-33 and the BH-21R which was a racing variant and was powered by a boosted Hispano-Suiza 8fb engine delivering 400hp of power.

It wasn't proven as it was retired just before the beginning of the World War II, however it served as a pivotal point for the BH-33 and BH-34 which much more advanced fighter types. The BH-21R earned some renown as it was the winner of several racing competitions in 1925.

Sources:
1. https://en.wikipedia.org/wiki/Avia_BH-21
2. Salamander Books - The Complete Book of Fighers