I disagree. I find B-26 the toughest buff to kill. From my experience, this is caused, mainly, because it turns very well, and is very fast. Bearing in mind the rest of the planeset, I think its flight model is right. The reasons why it is a formidable killer related, indirectly, to that. The ability to steer the plane from gun stations, plus the absence of G-force effects on the gunner makes the gunner job FAR too easy on any Buff.
Add to that the difference in lethality any buff has over any fighter(except Ju88) both in terms of range, ROF, ballistics and stability as a gun platform, and you got it. A fast, manouvering plane, extremely stable gun platform, not affected by G-forces (on the gunner task), and able to point each and every gun able (some times out of the real firing arc) to see the target, even through surfaces, to a single one, even in a multiple bogey environment.
Cheers,
Pepe