T28 Superheavy Tank
If you thought the Germans were the only ones that could field mammoth tanks, you clearly have never heard of the infamous project "T-28", which in all honesty was on-par with the mammoths of the German land forces.
T-28-95 super-heavy American tank
As world war two was appearing to be a similar standstill on all fronts to World War One, American tank designers began thinking like the English during that first bitter war , they needed something thick !
With tanks like the panther and tiger prowling around , the idea of sending swarms of ill-equipped M4 Shermans seemed doomed to fail. With the coming of the tiger II , the Sherman could no longer even penetrate many of the German armored units, let alone storm to Berlin.
Similarly in Japan , the Sherman appeared to be getting bogged down and destroyed by anti-tank weapons with relative ease (by an army that barely used other tanks).
So, by 1945 and prior to the fall of Berlin, the idea of a 300mm (one foot) thick mobile bunker, armed with a 105mm cannon made it to the drawing board. The idea was to basically drive this thing strait through all German defenses (at 8 mph), and to counter the tiger II-- which would have unfortunately probably flanked these easily in reality; destroying them.
By the middle 1945, the idea was changed around (following the fall of Berlin) , and the tank was redesigned as the t28/95 , wielding a larger cannon (120mm) and gaining significantly more side Armour along with a .50 cal machine-gun for dealing with pesky infantry -- with the intention of dealing with the Japanese.
After Japan surrendered in 1945, the tank was stopped from development and temporarily forgotten. Because the Soviet Union already had monster tanks of their own (according to British and American intelligence) , the t-28/95 continued to be developed from mid 1946 (idea re-instated). By 1947, multiple fully functioning models were completed , but the idea would later be dropped due to the inability of producing a half-decent suspension for the vehicle (early 1948) and main battle tanks were seen as "the future" of armored combat.
There is actually a video of this behemoth right here. Notice how the narrator explains the uselessness of this bunker buster against enemy Armour, although the 105 mm could penetrate a tiger II with relative comfort.
Through speculation it might be fair to assume that had the thing been built, it would have broken down less than 5 miles from the start in Europe or the far east. This tank was so heavy that it used two sets of large tracks , just imagine it in the swamps of the Ardene or the mucky rice patties of east china. Although on the open battlefield, it might have been a hell of a spectacle to observe.
t-28-95 completed at the Patton museum, USA