During the next two years, various prototypes were built under the designations T20, T22, T23, T25 and T26. These covered a variety of combinations of weapons, transmissions, and suspensions. However, the initial success of the M4 led the Army Ground Forces command to believe that there was no urgent need for a new tank. Even with the appearance of the heavy Tiger and medium Panther tanks, the AGF did not alter its position, believing both tanks would be fielded in relatively small numbers. AGF was correct about the Tiger, a specialized heavy tank that was never encountered in large numbers. The Panther, first encountered in small numbers at Anzio, however, was built in very large numbers and formed half the German tank strength in Normandy. Also, according to the Army doctrine of the time, tanks were not supposed to engage other tanks; this was the remit of tank destroyers, more mobile armored vehicles with powerful guns, such as the M10 Wolverine. As a result, the development of the new tank was slow. When the Allies invaded western Europe during Operation Overlord in June 1944, the M4 still formed the bulk of their tank units. It quickly became clear that the tank destroyer doctrine failed in the field and that the upgunned Sherman was unable to engage the Panther on equal terms. Efforts were made to speed up development but the tank, by now called the T26 and dubbed Pershing, only reached the battlefield in February 1945 and saw very little action in WWII.
In May 1946, due to changing conceptions of the US Army's tank needs, the M26 was reclassified as medium. Designed as a heavy tank, the Pershing was a significant upgrade from the M4 Sherman in terms of firepower and protection. On the other hand, its mobility was unsatisfactory for a medium tank (it used the same engine that powered the M4A3, which was some ten tons lighter) and its transmission was somewhat unreliable. In 1948, the M26E2 version was developed with a new powerpack. Eventually the new version was redesignated as the M46 General Patton. Thus the M26 became a base of the Patton tank series, which replaced it in early 1950s. The M47 Patton was an M46 Patton with a new turret, while the later M48 Patton and M60 Patton were completely new tank designs.