High powered SPGs came up again in late 1939. While storming the Mannerheim Line, the Red Army needed an SPG with a high caliber gun and at least light anti-shell armour. In the middle of December of 1939, the Kirov factory and factory #185 design bureaus received an order from the Military Council of the North-West Front to develop engineering tanks with anti-shell armour. Factory #185 started two projects at once. Work began on an SPG based on the T-100. Additionally, it was decided to equip the SU-14 with armour. You would assume that the already made SPG was the faster solution, but no, work only competed on March 20th, a week after the end of the Winter War. The creation of an SPG on the T-100 chassis was not fast either, resulting in a vehicle named T-100Y, armed with a 130 mm B-13 naval gun.
The Kirov factory took a different route. In January of 1940, a prototype of the KV heavy tank (serial number U-0) arrived from the front. According to orders from the Military Council of the North-West Front, the first 4 tanks, including U-0, were to be equipped with 152 mm howitzers to defeat pillboxes. An enlarged turret was rapidly developed as a result. Work on the enlarged turret was done by SKB-3, headed by N.V. Kurin. Initially, the 152 mm model 1909/30 howitzer was planned, indexed L-21. In metal, the assault KV was equipped with a shortened M-10 (indexed MT-1). By February 10th, the new U-0 passed gunnery trials, and by February 17th, U-0 and U-1 were sent back to the front. "KV with large turret" tanks first saw battle on February 22nd. By March 3rd, there were 4 tanks of this type on the front lines.
Due to the results of the Winter War, it was decided that the "bunker buster" theme would be continued. During the spring and summer of 1940, the Kirov factory and factory #185 continued designing heavy SPGs on the T-100 and SMK chassis. However, the final fate of these vehicles was decided by the end of June. Based on the results of trials, it was clear that neither tank will be mass produced, since the KV was more protected and weighed less. The "KV with a large turret" was an adequate interim solution for the bunker buster problem.
As mentioned above, the "KV with a large turret" was a temporary answer to the need for a heavy assault SPG. Kurin's tank was a hypertrophied support tank like the artillery BT-7 (frequently called BT-7A, but that index belongs to another tank). The vehicle on the KV chassis (indexed KV-2 in 1941) had the advantage of parts commonality with its base vehicle. However, it had many drawbacks. The chassis allowed for a limited power gun, while the military demanded that the Br-2 gun be used. Due to the limited size of the turret, loading the M-10T was no easy task. The presence of a rotating turret did not mean that the tank could shoot from any angle.
After the SMK and T-100 projects died, efforts were concentrated on the creation of a heavy tank that was a modernized KV. On July 17th, 1940, the Committee of Defense of the Council of People's Commissars issued decree #198ss on the creation of new vehicles on the KV chassis. According to the decree, the Kirov factory was to develop the following:
- Two experimental KV tanks (T-220) with 100 mm armour. One was to be armed with the F-30 85 mm gun, the other with the F-32 76 mm gun.
- Two experimental KV tanks with 90 mm armour. One was to be armed with the F-32 76 mm gun, the other with the F-30 85 mm gun.
- One experimental SPG with a Br-2 152 mm gun.