The B-25C/D had always been deficient in defensive armament, and the planes of the 321st Bombardment Group had 57 of their B-25Cs and Ds fitted with additional guns before being despatched to Africa. This work was done at the Warner Robins Air Logistics Center in Georgia. The virtually useless retractable ventral turret was removed and the hole in the floor left by the vacant turret was structured over. Larger windows were cut into the aft fuselage and a single 0.50-inch waist gun was mounted behind each window. A single flexible machine gun was installed in the extreme tail operated by a gunner sitting underneath a raised canopy that was somewhat similar to that later installed on the B-25H and J versions.
Additional airplanes were modified at the Sidi Ahmed Air Base at Bizerte. In addition to the extra defensive guns, more armor protection was provided, with plates being added underneath the pilot's seats, forward of the instrument panel, on the waist gun floor, and on the floor and sides of the tail gun position.
More than 300 B-25Cs and Ds from the 12th, 310th, 321st, and 340th Bombardment Groups were processed. These planes served throughout the Mediterranean campaign from Tunisia to the final German surrender in northern Italy. Many of these modifications were later introduced on the production line with the B-25H and J versions.