I think that the problems from the B-26 stemmed from its high landing speed...about 130mph. That was fast for a plane of its day. Also, it had some funny handling quirks and in certain conditions it was not an easy plane to fly.
Also, when it was empty, it had to have ballast in the back of the plane to land it properly. Otherwise, you could easily pound the nose gear into the runway too hard, damaging it on a concrete runway and jamming it into the ground on a dirt runway causing it to flip end over end.
I do not recal a modification on the wings once it went into production, except to possibly put some hardpoints on them to carry two extra 500 lb bombs. It had a total bomb load of 4000 lbs. That was a lot for the time.

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