The B-23 Dragon started out to be a medium bomber to replace the B-18, but it became an all crew trainer.
The resulting XB-23 had a cleaner fuselage and added more defensive weapons. As the speed would be above 280 mph it was felt a nose gun wasn't necessary. The first ever tail gun, for a United States bomber, was installed.
By 1939 the Army was going to purchase the B-17 so the B-23 was submitted in the medium bomber Circular Proposal 39-640. It finished 3rd and wasn't picked for mass production.