The M55 is an American heavy self-propelled howitzer (SPH) developed in 1950 as part of the M53/M55 project to replace the M40/43 HMC and fulfil the long-range artillery role. While both vehicles shared the same modified M47 Patton chassis, they were armed with different guns to evaluate the effectiveness of their roles: the M53 was armed with a 155 mm M46 gun for long range, while the M55 was armed with a 203 mm M47 howitzer which maximised firepower at the cost of low ammunition capacity. After firing tests in 1952, it was decided that the M55 offered more strategic advantage over the M53, and it was accepted into production that year. The M55 saw service primarily during the Vietnam War as an anti-fortification weapon, and would serve in this capacity until 1969, when it was replaced by the M110 SPH. In total, 550 M55s were built, with some supplied to US foreign partners as part of military assistance programmes. Among the countries that received these vehicles were Belgium, West Germany, Italy, Spain, Turkey, and Taiwan.

Introduced in Update "Tusk Force", the M55's main (and only) strength is its firepower: a massive 203 mm M47 howitzer, firing an incredibly powerful HE shell with approximately 23 kg TNTeq of filler. This shell is capable of destroying any tank in the game when aimed properly. However, one needs to make sure that every shot counts, as the M55 carries only 10 rounds of ammunition and the howitzer has a very long reload time. Missing a shot leaves the vehicle at the mercy of the enemy, who will be more than eager to poke holes in its large, thinly clad hull.