I've read accounts that it was a mixed bag. When it worked, it was great but it had other issues. Weight, reloads, other stuff I've forgotten. That led to the "strafer nose" with 8 .50 cals, plus 4 in cheek/ mounts. Numbers differ slightly, but crews would boresight all of those to converge at a known distance and could "cut a ship in two." As a kid I thought that was cool but too good to be true. Later there are indeed accounts of B-25's doing just that to smaller shipping. Hyperbole from the aircrews? I don't know.
12 .50 cals plus 2 in the upper turret for 14 total is a lot of freaking lead. With parafrags and skip bombing, B-25's and A-20's plowed through the south Pacific with the 5th (?) Air Force. They and B-24's almost exclusively pinned Rabaul in place and allowed it to be bypassed.