The caleidoscope of various vehicles based on the Heavy Tank T26E3 shows how difficult the situation was with American heavy tanks. The T26E3 was really a medium tank, especially when it came to armour. Several variants were developed to make it into a true heavy, from installing more powerful armament to a thorough modernization that resulted in the Heavy Tank T32. The simplest variant was the Heavy Tank T26E5 where the designers decided to limit themselves to thickening the tank's armour.
American medium tanks inevitably gained armour as they evolved. The Medium Tank T20 had just 64 mm of front armour in the hull and 89 mm in the turret. With the T26E3 this armour increased to 102 mm. This was more than most medium tanks of the period, but with the caveat that the cast armour had lower toughness than rolled. However, the T26 was moved to the heavy tank class in the summer of 1944 entirely due to its growing mass. In reality it was still a medium tank, albeit from a new generation. Protection didn't magically increase when its designation changed, but as the evolution of the Heavy Tank T29 and Heavy Tank T30 later showed, American generals considered 102 mm of front armour sufficient for a heavy tank.