https://en.wikipedia.org/wiki/M3_Lee
The design was unusual because the main weapon – a larger caliber, medium-velocity 75 mm gun – was in an offset sponson mounted in the hull with limited traverse. The sponson mount was necessary because, at the time, American tank plants did not have the design experience necessary to make a gun turret capable of holding a 75 mm weapon.
Good context. All true, the M3 was indeed kludged together in a very short time after America didn’t invest a lot into interwar tank design.
In addition, the configuration of putting the main gun in the hull was not unprecedented. The French Char B1 did the same thing, and it was considered a very capable tank, even by the Germans up against in in 1940.
If you put yourself in the mindset of a 1930s designer trying to figure out the role of a tank and assuming a kind of breakthrough role, it does make sense to put the lighter weapon meant to target more mobile targets on a traversing turret and the heavier weapon for targeting heavy defensive positions in the hull rather than trying to figure out how to fit such a large gun into a turret.
