In 1935, the Wehrmacht, which was in a phase of rearmament, requested from the German heavy industry weapons of uncommon characteristics. The "Dora" super cannon, for example, can not be actually considered to be proportional to the war material of normal utilization. In this case it would be easy to guess that it was not an ordinary cannon, but one which had been designed for firing against a very specific target: the Maginot Line. A weapon tailored for a particular necessity. The same year a gigantic piece of siege artillery was ordered: a 600-millimeter mobile mortar, capable of moving through its own means to take positions against the targets which, because of their distance from the railway lines, could not be reached by cannons installed in railway carriages. If we consider the taste that the German have always shown for this type of artillery, and if we think that in 1935 mobile artillery, in the true sense of the word, had not been yet born, this unexpected necessity from the Wehrmacht of a weapon of this genre seems, at least, strange. However, if we look at a railway map of the Europe of those years, we see that the borders of the western nations and also the French coast of the Channel, the wide natural frontier of England, are crossed lengthwise and crosswise by railway lines. To the east, however, things are different: the Russian railways, for example, were few, bad and, almost all of them, built with a gauge different from the European one; hence, they were useless for an eventual military utilization. Besides, Russia possessed a good fortified line and many strongholds which were, on their largest part, safe from railway artillery. Thus, the German required and built artillery of large caliber which did not rely on such communication lines for its operation. But this symptomatic alarm bell was not noticed by anyone, or else nobody wanted to hear it...

In the early 1939 the Rheinmetall-Borsig company was able to present the prototype of a steel mastodon, denominated "Karl", which weighed 123 tonnes and with which the trials and ballistic tests were immediately effectuated. Barely two years later, in 1941, the first two series weapons, denominated "Thor" and "Eva", were already finished. They were gigantic mortars, capable of shooting either piercing or high-explosive projectiles, of truly devastating effects, because of both their own weight and the steep angle with which they fell, characteristic of mortar fire. However, it was thought that their range was not satisfactory and thus it was studied the installation of a cannon of lesser caliber (540 millimeters), but with external dimensions matching those of the 600-millimeter cannon, so that both could be installed in their position, according to the requirements, without requiring any modification. The lesser caliber, as well as the length of the cannon, allowed to considerably improve the range. Besides several of these cannons, another four tracked tractors were built. The engine, a Mercedes-Benz, could be installed either as Diesel or gasoline version, but the first type was generally preferred. Before opening fire, a gear system allowed to block the suspension and descend the body of the carriage, making it to rest directly on the ground. This prevented that the transmission mechanisms were damaged by the recoil. For short displacements, the "Thor" used its own engine, whereas for longer travels the tractor was loaded on a special towing and the cannon on another one. Both were then towed by artillery tractors. For very long travels transportation through railway was used instead. The "Thor" could be loaded, by means of a special clutch, on two platform wagons.