The Landkreuzer P.1000 Ratte and P.1500 Monster were the largest tanks ever designed in history and they were first thought up in Germany.
In order to power the tanks, eight Daimler Benz MB501 Engines or two Man V12Z32 Engines would be required for each tank.
Naturally, they were scrapped due to production costs by the Ministry of Armaments. However, the P.1000 would have weighed 1000 metric tons while the P.1500 would have weighed about 1500 metric tons, thus giving them their designations. Both would have made easy prey for Allied bombers, like the B-17 and Lancaster, because they were going to be extremely slow.
To counter this, the P.1000 equipped eight FlaK 38s. They would, however, easily destroy Allied fortresses and tank formations in which if they could be defended by the Luftwaffe, might eventually make up for the enormous production costs. Besides the AA armament, the P.1000 also mounted two 280mm main cannons, a single 128mm KwK 44 and two defensive machine guns. The crew required to man such a tank would have been around 42 men.
The length of the tank alone would have been about 35 meters while the width would have been about 14 meters had the tank not been scrapped. When the P.1000 Ratte was scrapped, only the turret, and a few other parts were completed. This unfinished turret was used as a stationary mount later in the war.
The P.1500 was supposed to have 250mm thick armor and mount the 800mm K.(E) Rail Gun along with two 15 cm sFH 18 Howitzers and large amounts of defensive MG 42s. The crew required to man the tank would have numbered about 100 men. When the P.1500 Monster was scrapped, only the tracks were completed. Those tracks were later cut into smaller tracks and used on Tigers and Panthers.
As mentioned above, the only parts of the P.1000 Ratte that was finished were the turret and some other sections, but for all intents and purposes, no real prototype was created. The same could be said for the P.1500 Monster.
The idea the P.1000 Ratte and Monster was first conceived in 1942 by the Krupp company, known for their extra-large building projects. Hitler had expressed great personal interest in the projects and so construction of the tanks was initiated, but was soon canceled because of the monstrous amounts of resources that would be required to make such tanks. One of the true signs that the project was doomed to failure was the performance of the Panzer VIII Maus during testing. It guzzled fuel and it completely destroyed most roads that it traversed. Also importantly, it would destroy almost every bridge it tried to cross. Rail was the most viable option yet rail roads were constantly terrorized by allied "jabos" or ground attack fighters.