Hajn was first a founder and chief designer at Avia, along with Pavel Beneš, in 1919. The two began repairing planes in a workshop within the complex of an old sugar factory in Prague. One year later, they designed their first two-seater plane, the Avia BH-1. From 1923 to 1925, the two developed the Avia BH-7, BH-9, and BH-11 monoplanes, launching the era of biplane fighters. The BH-11 won the Coppa d' Italia prize. Three years later, their Avia BH-21 fighter was considered one of the world's best planes.