In December 1942, the Engineering Division of the Air Force Materiel Center directed that the possibility of fitting two 37-mm cannon into the B-25 be explored. In response, North American prepared a brief proposal which involved the installation of two 37-mm cannon in a deep "bathtub"-type fairing installed underneath the bomb bay. The aft section of the fairing carried a bay for a battery of parafrag bombs, the bay blending smoothly into the ventral turret. Additional armament was to have included a pair of fixed 0.50-inch machine guns in the nose, a 75-mm cannon in the nose tunnel, two fixed 0.50-inch machine guns in a module mounted underneath the forward hatch, and a pair 0.50-inch machine guns in both aft turrets.
B-25C 41-12800 was received by North American in February 1943 for use as a test aircraft for this project. Unfortunately, tests proved that structural damage from the blast of the 37-mm cannon was so severe that major structural changes to the airframe would have to be made before the concept could be made to work. The project was abandoned before any further conversions could be carried out.