The Soviet Union invaded neighboring Finland to spark the "Winter War" (1939-1940) and, despite resilience and ingenuity by the Finns, territorial changes ensued as did the Moscow Peace Treaty. A new war erupted between the two parties in June of 1941, the Finns now backed by the strength of Berlin. This subsequent war became known as the "Continuation War" which spanned from 1941 until September 1944.
During this renewed fighting, the Finns began to mate hulls of captured Soviet Army BT-7 (Model 1937) series light tanks with British QF 4.5" field howitzers, these guns having been received from the British government during the Winter War period. The intent was to produce a viable assault gun by combining two proven weapon systems. Resources available to the Finns meant that only eighteen of the kind were completed and these vehicles were designated as "BT-42".
The original Soviet drive components of the BT-7 were retained and this included a Mikulin M-17T series engine of 500 horsepower output. This allowed for speeds of 33 miles per hour to be reached with operational ranges out to 235 miles. The Christie suspension system offered the needed off-road support for cross-country traveling. The crew numbered three and the vehicle weighed about 15 tonnes. In appearance the BT-42 held onto most of the original BT-7 form and function from the hull roof on down. An all new turret was designed and constructed in which the 4.5" weapon protruded, along with its recoil mechanism, from the front panel. Traversal was a full 360-degrees giving the crew some tactical flexibility.
BT-42 tanks were shipped to the front in 1943 with first combat taking place at the Svir River against enemy fortifications. It proved lacking against armored targets, particularly the new Soviet T-34 Medium Tank and even a special Anti-Tank projectile was designed which failed in its intended scope. It was also found that the running gear of BT-7 tanks were not all that reliable and the engines whined under the stresses of the heavier turret and armament. The turret also added to the vehicle's profile which made it an easier target for AT crews and tanker crews at range. All that said, the BT-42 was an available weapon system during a time when the Finnish army needed anything it could lay its hands on to stave off total elimination.
Final operational use of BT-42 systems was in 1944 for, by this time, more modern and heavier German equipment was becoming available to the Finnish Army. Nevertheless the Continuation War ended in a Soviet victory and Berlin was beaten to end the war in Europe by May of 1945.