Several competing designs were offered, and even more were drawn up prior to reaching prototype stage. All had heavy armour,torsion-bar suspension, wide tracks, and were of welded and cast construction. One of the main competing designs was the SMK, which in its final form had two turrets, mounting the same combination of 76.2 mm and 45 mm weapons as the T-35. The designers of the SMK independently drew up a single-turreted variant and this received approval at the highest level. Two of these, named after the People's Defence Commissioner were ordered alongside a single SMK. The smaller hull and single turret enabled the designer to install heavy frontal and turret armour while keeping the weight within manageable limits.
When the Soviets entered the Winter War, the SMK, KV and a third design, the T-100, were sent to be tested in combat conditions. The KV outperformed the SMK and T-100 designs. The KV's heavy armour proved highly resistant to Finnish anti-tank weapons, making it more difficult to stop. In 1939, the production of 50 KVs was ordered. During the War, the Soviets found it difficult to deal with the concrete bunkers used by the Finns and a request was made for a tank with a large howitzer. One of the rush projects to meet the request put the howitzer in a new turret on one of the KV tanks.
Initially known as Little Turret and Big turret, the 76-mm-armed tank was designated as the KV-1 Heavy Tank and the 152 mmhowitzer one as KV-2 Heavy Artillery Tank.
The KV's strengths included armor that was impenetrable by any tank-mounted weapon then in service except at point-blank range, that it had good firepower, and that it had good traction on soft ground. It also had serious flaws all of which were rectified with the introduction of the KV-1S: it was difficult to steer, the transmission (which was a twenty-year-old Caterpillar design)was unreliable (and was known to have to been shifted with a hammer), and the ergonomics were poor, with limited visibility and no turret basket. Furthermore, at 45 tons, it was simply too heavy. This severely impacted the maneuverability, not so much in terms of maximum speed, as through inability to cross many bridges medium tanks could cross. The KV outweighed most other tanks of the era, being about twice as heavy as the heaviest contemporary German tank. KVs were never equipped with a snorkelling system to ford shallow rivers, so they had to be left to travel to an adequate bridge. As applique armor and other improvements were added without increasing engine power, later models were less capable of keeping up to speed with medium tanks and had more trouble with difficult terrain. In addition, its firepower was no better than the T-34. It took field reports from senior commanders "and certified heroes", who could be honest without risk of punishment, to reveal "what a dog the KV-1 really was."
While initially the Soviets made a lot of poor defense decisions, worsened by recent "cleansings" of Soviet military command, the KV-1 was unlike anything the German army had expected to encounter, and some of the battles against numerically superior Axis forces became legendary. Even though the operations of the KV family of tanks were severely hampered by restrictions due to its weight, they were fearsome and formidable weapons through most of the Second World War.