The BT-42 was developed on the chassis of captured Soviet BT-7 obr 1937 tanks. The standard turret BT-7 was rebuilt to mount the British Q.F. 4.5inch Howitzer Mark II (114mm, Finnish designation 114 Psv.H/18) donated to the Finns during the Winter War. The howitzers additionally were fitted with a Finnish-designed muzzle brake.
The BT-42 saw its first combat at the Syväri (Svir) River in 1943. They were used to bombard Soviet fortifications on the opposite side of the river. They used both direct and indirect fire, and though they destroyed several bunkers, the Finnish crews were not impressed with the new vehicle.
The BT-42 was prone to mechanical failures inherited from its BT-7 hull and further exacerbated by the added weight of the howitzer and larger turret.
In summer 1944 the BT-42 saw its next action in and around Viipuri (Vyborg) against a Soviet offensive. They lost eight BT-42s during the fighting to defend the town. During the fighting, the inadequacy of the 114mm gun against armor was proved when one BT-42 hit a heavy Soviet tank 18 times without causing any noticeable effect on the tank. Soon after the BT-42s were withdrawn from the field to be scrapped.