So he didn't kill that many planes, but rather got that many kills from the one formation? That would have been a good bit of info to include.
It's been ages ago, but I got something in the neighborhood of 200 kills off one formation of B17s. I think it's a very sporadic bug with the formations and trying to bail.
edit: scratch that, it's obviously a bug with attached gunners. Kid has 283 deaths as a gunner.