Why wasn't there a belly gunner on the B-25 Mitchell?
The production B-25B, B-25C, B-25D, and some B-25G models did have retractable remote control belly turrets. They were often removed in the field because they were ineffective and disliked by the crews. 

The lower turret was officially deleted in the middle of the B-25G production run and continued with the B-25H and B-25J production.

More info on the B-25 here: B-25 History.org