It worked so well, the factory started producing cannon equipped models
The B-25G came with a cannon from the factory along with several .50 caliber machine guns.
It was a potent combination. From the article, "By the time of the Marshall's campaign of early 1944, worthwhile targets for the 75-mm cannon had become scarce. By this time, there were fewer targets that could not be dealt with more effectively by machine guns or bombs. Consequently, the big cannon was often removed from the B-25G, with a pair of additional 0.50-inch machine guns being fitted in the nose tunnel where the cannon had been located. A total of 82 B-25Gs were so modified at the Townsville Depot by April of 1944."