The armament varied during the production run.
http://home.att.net/~jbaugher4/a20_17.htmlA-20G-1-DO (250 built)
Nose: 4 x 20 mm fixed
Forward Fuselage: 2 x .50 cal fixed
Dorsal: 2 x .50 cal flexible
Ventral: 1 x .30 cal or 1 x .50 cal flexible
Bombs: 2000 lb internal
A-20G-5-DO to A-20G-15-DO (500 built)
Nose: 4 x .50 cal fixed
Forward Fuselage: 2 x .50 cal fixed
Dorsal: 2 x .50 cal flexible
Ventral: 1 x .30 cal or 1 x .50 cal flexible
Bombs: 2000 lb internal
A-20G-20-DO to A-20G-45-DO (2100 built)
Nose: 4 x .50 cal fixed
Forward Fuselage: 2 x .50 cal fixed
Dorsal: 2 x .50 cal Martin turret
Ventral: 1 x .50 cal flexible
Bombs: 2000 lb internal, 2000 lb external
Screenshots of the AH A-20G show a Martin turret, so it's an A-20G-20-DO or later.