The B model is an earlier variant, saw action during the Phony War, Battle of France and Battle of Britain. it is also the one that most think of when they talk about the Stuka.
The G-1 is the one with the 37mm Flak 18 cannon that is built around the D-3 variant air frame.
The D-3 is considered to be the crappiest of the lot due to the increased weight (due to added armor) the wing loading became unacceptable and was replaced by the D-5 version.
The G-2 is the same as the G-1 with the sole exception that is was based on the D-5 rather than the D-3.
The D-5 had the extended pointy wingtips that improved handling over the D-3 model, and had the wing mounted machine guns replaced with 20mm cannons. The dive flaps were also removed on this model.
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