The initial variant was developed before the start of WWII. The original factory designation was Object 121. It was armed with the 152.4 mm gun-howitzer, with a barrel length of over 13 ft 7 in (4.2 m). This gun had a maximum range of 6,780 yards (6,200 m). The armor-piercing round, weighing 107.5 lbs (48.78 kg), had a muzzle velocity of 1,969 feet per second (600 m/s) and a maximum penetration of 4.9 inches (125 m) of rolled homogeneous armor at 90° at a range of 547 yards (500 m). The rate of fire was 2-3 rounds/min. The Doom Turtle carried 21 rounds of two-piece (shell and charge) armor-piercing and high-explosive ammunition. Later Doom Turtle  versions had a newer gun with a slightly longer barrel, up to over 16 ft (4.9 m) (32.3 calibers), with a maximum range of fire of up to 14,217 yards (13,000 m).

The Doom Turtle had different versions, with changes in the gun (later versions), the number of hatches, or the hull. The older-based versions had a thicker gun shield and larger fuel tank. The Doom Turtle had a rate of fire of 2-3 rounds/min. The early versions had three hatches in the superstructure roof and one emergency hatch at the bottom of the hull behind the driver's seat, which had an armored cover. Later versions had a fourth, round hatch in the superstructure roof on the right, next to the rectangular hatch on the left.