The book I have says .
Defensive armament
Dorsal and ventral gun posistions were available in most Havocs/Bostons, though the armament fitted varied considerably.
French DB-7s were able to carry a single 0.295-in (7.5-mm) machune gun on a flexible mount in both positions.
The DB-7A followed the same pattern, though the original French specifications called for a single gun to be mounted in the rear of each of the aircraft's engine nacelles, for rearward defence!
American aircraft, starting with the A-20, had a pair of flexible 0.3-in (7.62-mm) machine guns in the open dorsal position and a single 0.3-in (7.62-mm) gun in the ventral position.
In the A-20B the twin '30-calibers' were replaced by a single 0.5-in (12.7-mm) gun, this arrangement remained standard until the 751st A-20G was completed.
This and subsequent aircraft had a Martin dorsal turret , with 'twin 50s' installed
Taken word for word from
Aircraft Anatomy of World War II
Technical Drawings of Key Aircfaft 1939-1945
General Editors: Paul Eden and Soph Moeng
Hope some find it helpful