In order to add more firepower to their armored brigade – mostly equipped with obsolete T-26s, the Finnish army resorted to indigenous improvisation. Finnish troops had captured a plethora of BT-7s which couldn’t be put into use as such, but they were erroneously deemed suitable as platform for a new improvised assault guns, the BT-42. In total, 18 captured BT-7s were converted to BT-42s from 1942 onwards. In Valtion Tykkitehdas, they were fitted with a new turret, that shared some likeness with the dreaded KV-2, although it could be only barely called armored.
In service, the combination of the ancient gun, the high profile and the thin 15mm armour of the turret meant that the BT-42 was a dismal failure when pitted against Soviet armour. In a single engagement on 9 June 1944, the defence of Vyborg, eight of the nine BT-42s deployed were lost. This disastrous action saw the disbandment of the Finnish Detached Armour Company.