By the time the M2A4 entered production, the war in Europe was raging on for six months. Naturally, the Americans were watching closely and making their own conclusions. For example, it was clear that armour which can only protect from large caliber machineguns and autocannons was not enough. In Poland, the main killer of tanks was already anti-tank cannons. The campaign in France only highlighted the fact that tank armour needs to grow. In addition, tanks armed with 20-25 mm cannons were the minority in the French campaign. Thick armour was becoming more and more important. Recall that Gladeon M. Barnes proposed 1.5" (38 mm) thick armour back in 1938.
The Light Tank M2A4, despite its outstanding characteristics, had many drawbacks. The length to width ratio was small, and if it was not a problem with 8-9 ton tanks, then the 12 ton M2A4 had a tendency to rock back and forth. The solution was simple: lengthen the contact surface between the tracks and the ground. The design of the observation devices was also not the best, and the massive gun mantlet had its weak spots.