As production of the North American B-25 Mitchell progressed throughout the early years of WWII, the arrangement of its weaponry changed dramatically with ensuing models, and the aircraft were further modified out in the field by crews wanting yet more firepower. Quite often the plane was used as a ground attack weapon rather than a medium-altitude bomber, and this strongly motivated the addition of more and more machine guns, especially those which faced forward. The MojaveWest archive recently acquired three 8x10 images that originally were taken by official North American Aircraft photographers of various machine gun installations on B-25s.
The image of the left shows a standard B-25C/D installation of two .50 caliber machine guns, one movable that was fired by the bombardier, and one fixed which was fired by the pilot.
The image on the right is a bit more unusual, as it shows an installation of two guns in a configuration that appears to have been experimental at NAA, but which didn't make it to production. While later models, including the B-25H, included twin-.50 caliber guns, they were set much farther apart and were fixed in place. These appear to be movable, and designed to both be fired by the bombardier. I have yet to come across any other images showing this configuration, so if any of you has further information on this particular installation, I'd love to hear from you in the comments section below.
The image of the left shows the two .50 caliber waist guns installed on a B-25J. While earlier models included waist guns, the -J model introduced a more functional installation, allowing a greater field of fire from the movable guns. Also note the N-8 gun sights which were also introduced with the -J model.