The first 555 B-25Js were delivered without the fuselage package guns, because it was discovered that muzzle blast from these guns was causing severe stress in the fuselage; this problem was cured with heavier fuselage skin patches. Although later production runs returned these fuselage package guns to the aircraft, they were often removed as a field modification for the same reason. In all, 4,318 B-25Js were built.