By the way, there was no real B-25 prototype (XB-25/YB-25, etc). They just started at B-25. That's a bit of trivia most people don't know about the way US designations used to work. They didn't start at -A; they started at
sans suffix. The B-25A was actually the 2nd model built or, if you'd rather, the first
variant (which is really what the letters noted). For all practical purposes, the first production models are generally -A models, since they're a
variant of the XB-## prototype or YP-## pre-production aircraft. This was eventually changed for a number of reasons, not least of which being that 'B-25' in this instance can refer to the specific model that were those initial 24 aircraft were constructed as or it can refer to the whole family. It's ambiguous, but it's used correctly in either case.
Also, there wasn't anything inherently
wrong with the first B-25s as aircraft. As bombers, however, they did suffer from a lack of stability. Since they also lacked armor and self-sealing fuel tanks, they were unsuitable as combat aircraft. Instead, they were used as training aircraft. I've read that the first Soviet pilots trained on the B-25 in the US were trained on the initial production models with full-span dihedral, no armor, etc. They had been initially trained as fighter pilots in the VVS and--to their immense delight--those initial B-25s handled quite like large fighters. There were a lot of issues with Soviet pilots tossing the planes around the sky in ways no one ever intended. They'd land with rivets in the tail missing and tail gunners with cracked ribs.
In one case, US fighter pilots were teaching them evasion techniques as they flew P-43s against them in mock combat. The Soviet fighter pilots in B-25s were nonplussed and quickly got on the tails of their would-be pursuers, even forcing a P-43 into the ground in low level air-to-air combat!That should give you some idea of just how "hot" of an airplane those initial B-25s were!
Cheers,
Logan