In autumn 1941, the Finnish Army captured 20 operational BT-7 Soviet light tanks, and by spring they had started modernizing and upgrading them into SPGs. The first models were built in 1942, and the new vehicle was designated BT-42, with a total of 18 tanks converted from 1942 through 1943. The turret underwent the most significant changes. It was armed with a 4.5-inch (114 mm) British field howitzer produced in 1909–1917 featuring a gun mantlet and muzzle break of Finnish design.
These assault vehicles first saw battle in spring 1943, when the Finnish Army attacked Soviet positions on the south bank of the Svir River. As of January 1, 1945, the Finnish Army had a total of nine BT-42 vehicles, but by that time they had been removed from service and sent to training units. In the late 1950s, the remaining vehicles were retired from the Army and scrapped.