If the above rules apply.....then......
......... the T/28 / T95...arguably excluded....due to the time limit

...Wiki,... which I know can be unreliable says ' It was first conceived in the spring of 1945, but proved to be
too late to be used in World War II. The original name for the project was to be T28. The Pacific Car and Foundry Company designed it for the final push in Europe,
but by the time the first tank was completed and ready for combat, the war was over.'
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Tzar tank is Definately out as it was built in 1915 !!
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I am not sure if the Landkreuzer P. 1000 Ratte or the The Landkreuzer P. 1500 Monster qualify as they were
only paper exercises and were cancelled by Albert Speer.
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The Panzer VIII Maus ....However
WOULD qualify as several were actually made and found to be realistically simply too heavy to use.
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JS7 / JS 10...... also known as the ....T-10 after the death ( Hurrah) of stalin............TOO late to qualify ..built in 1948 and 1952 respectively....
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KV-VI Behemoth tank....qualifies......and I am Delighted to see that it was such a 'HUGE' success
Operational History
The first prototype was completed in December 1941 and was rushed into the defense of Moscow. In its first action during a dense winter fog, the rear turret accidentally fired into the center turret. The resulting explosion completely destroyed the vehicle.
The second prototype was completed in January 1942, and was sent to the Leningrad front. This one had indicators installed to show whe another turret was in the line of fire. In its initial attack on the Germans, the tank broke in half when crossing a ravine. A spark ignited the leaking flamethrower fuel and the resulting explosion completely destroyed the vehicle.
The third prototype, shown here, had a reinforced hull and was also sent to the Leningrad front in early 1942. It did manage to shoot down three German aircraft. In its first ground engagement, the KV-VI was firing on German positions when coincidentally all of the guns fired from the 3 O'Clock position a the same time. The tremendous recoil tipped the tank into a ditch and the severe jostling set off the 152mm ammunition, which completely destroyed the vehicle...

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