It wasn't a bomber at all, sorry to say. Bombers of the era, at a minimum, require a bomb bay and doors, and the P61 had neither. Perhaps you are confusing the presence of drop tanks with bombs, but it is easy to verify that it was designed and built as a night interceptor. It would have been totally inappropriate and useless as a bomber. Just because it was as large as some of the smaller period bombers and looks vaguely like one does not make it one.
The center gondola was filled with fuel and other equipment and the plane had no cargo capacity for bombs or anything else. Even if it did, the lack of bomb doors meant it couldn't have dropped a bomb to save its life.
Bombers had the B designation. The P designation marks it as a pursuit aircraft. So sorry, but the original commenter had it right and it was never used as a bomber. If you don't believe it there are any number of good online sources and five minutes with Google will confirm what I wrote.