Staff Writer (Updated: 12/19/2014):
Both projects were approved for development to fulfill a 1942 German Ministry of Armaments requirement. Like the P.1000, the P.1500 would come to nothing by war's end - canceled by then-Minister of Armaments Albert Speer as soon as 1943. Had it been completed, the P.1500 would of held limited battlefield value - its massive size restricting mobility across the varied terrain of the European countryside while also consuming much needed war material and manpower (operation of the vehicle would have required approximately 100 men).
At its core, the Landkreuzer P.1500 system would field a powerful 800mm K(E) main gun to be used primarily in countering fortified enemy positions. The primary armament was to be situated within a fixed superstructure which would have to be of considerable size and strength to carry the gun. The recoil equipment would be integral to the gun and its mounting hardware though have considerable tolerances for the forces at play. The powerful nature of the 800mm main gun would allow the system to operate clear out of the range of enemy fire and deliver a formidable payload at distance - quite similar to the concept behind the Imperial German Army's "Paris Gun" of a World War before. Additionally, engineers considered a pair of 15-cm (150mm) sFH 18.1 L/30 field guns for shorter-range assailing to go alongside the 800mm weapon. The platform - should it come under direct attack from the air - was to be defensed by a network of 15mm MG151/15 series autocannons mounted about the giant structure. Armor protection was to reach 250mm (nearly 10 inches) thick across critical facings of the structure.
Of course one of the greatest challenges facing engineers of the project was to be the power required to propel the 46-ton beast. This led to the prospect of having 4 x MAN M9v 40/46 diesel engines mated to the specially-designed tracked hull - the same marine diesels powering German U-boat submarine classes. Each engine was rated with an output of 2,200 horsepower which resulted in an estimated maximum road speed of no more than 9 miles per hour (on ideal surfaces). Operational ranges, though never formally estimated, was sure to be very limited and cross-country travel impossible. Considering the nature of European roads of the time and its narrow bridges, the Monster would have had a tough go of it when on the march. Rail-based travel was a possible alternative but engineers were driven by a more flexible, self-propelled solution - particularly as greater strides were being made with ever larger tank developments of the war that included the fabled "Tiger" and "King Tiger" heavy tanks coming online.
In the end, the P.1500 became yet another of Germany's abandoned fantastical wartime projects, limited to the minds and drawing boards of a few far-reaching men. The expected powerplant arrangement might have not supplied the required propulsion to move such a large and heavy weapon within acceptable time frames and its large battlefield profile would have made it near-impossible to conceal from the air - let alone defend it from concentrated air attack. Logistically, the P.1500 was simply too large a development to be of any tactical use and transportation/relocation of the system would be problematic for the life of the vehicle. Its large crew would have required support from a collection of vehicles to provide ammunition and general supplies which added to its operational oil, fuel, and munitions usage as well as requiring the services of specially-trained personnel to commit to a myriad of jobs while on the platform. As such, all development on the P.1500 ceased during 1943 with little work on the project actually having been completed. The P.1500 joined the P.1000 in never seeing the light of day, even in prototype/pilot form.
Other German super programs managed a more extended course - such as the super-heavy Panzerkampfwagen VIII "Mouse" tank - another of these famous secret mega-projects. Indeed it was manufactured in two examples (though one only partially) during 1944 but these vehicles only served to highlight the problems to be had in building extremely large and heavy tracked weapon systems. These pilot vehicles made her the largest enclosed armored combat vehicle ever produced in military history - one of the prototypes eventually falling to the invading Soviets in their advance on Germany.