B25 is an early/midwar bomber, B26 is a mid/latewar bomber.
B25 has 1.5k ord normal load (3x500lbs) or I think at most 2k ord max load (2x1000lbs), whereas B26 takes 4k (x3 bombers).
B25 -- if it had a solid nose -- would not have formations. B26 has formations.
Also B25 -- if it had a solid nose -- would have between 8x50cal and 12x50cal fixed and firing forward, later models carrying 4 rockets under the wings. It was a true ground attack craft. B26 has only 4x 50cal fixed forward and no rockets. Can't dive worth crap, loses elevators quickly, ailerons too. I can only hope/imagine the B25 is more sturdy to allow attacking things.
There's enough differences to warrant the B25. It was one of the most common bombers throughout the war.