Cs aren't nearly as defenseless as you might suggest. For whatever reasons HTC has modeled them as bullet sponges. I have experienced this as both attacker AND attacked. I've been in scenarios where entire flights of Fw190s ran out of ammo attacking B-25Cs and only getting 1 or 2 kills. I, personally, have withstood attacks from 3 Fw190D-9s in a single B-25C and survived to limp home 5 sectors on 1 engine. They broke off, having run out of ammo. The screenshots of the holes in my plane were epic.
The B-25 (C/H) aren't all that much of a bullet sponge, it really depends on where you hit them. Hit them aft of the cockpit and it will soak up some damage because the rounds are largely hitting areas with no vital, critical components. Hit near or forward of the cockpit, wing root (especially vulnerable part on the Mitchell), or engine and you'll be very surprised at how little ammunition it takes. It takes me no more than a solid burst at the wing root or engine to catch a Mitchell on fire and it takes me no more than 3 passes to take out a formation. Those FW 190s you ran up against were obviously not very good shooters.
I've also landed large numbers of 20mm hits (Fw190A8 with 4x20mm guns package) to no effect. Heck, I've hit B-25Cs with the spud gun on a P-39 point blank multiple times, and only oiled a single engine.
Again, it really depends on where you land your shots. I have no troubles with the 20mm cannon on the P-38 when engaging Mitchells, even the quad .50s are more than sufficient enough to down an entire formation.
Because of their absurd resilience, they are quite defensible. On a single sortie I convinced 2 squadmates to up 25C formations with me. I nailed a single Me262 that attacked up, and one of my squaddies got another. On another sortie, he got TWO 262s in a single sortie that attacked him.
Sure, the B-25C can defend itself but against a determined attacker that knows what he's doing, the B-25C doesn't have a snow ball's chance in Hell. This is the one bomber in which it is safe to engage from slighly below (to keep out of the top turret's line of fire) the dead six position and sit there plinking away at the Mitchell and there is nothing but that the Mitchell driver can do other than cringe and pucker up. Even coming from above, the top turret really isn't that much of a threat unless you are careless in your attack. Broadside attacks are also quite effective against the Mitchell.
As for the Me 262s that attacked you, all your anecdote does is tell me that you ran into a couple of Me 262 drivers that didn't know squat in attacking a bomber.
ack-ack