I do not think it was ever armed with cannons, just the 6 or 8 x 12.7mm
It was designed with a modular nose that could be equipped with a variety and mix of cannons and machine guns.

Combinations included a mix of .50 cal. mg plus 37mm or 75mm cannon, 2 37mm cannon, 1 37mm cannon and 1 75mm cannon, or all .50 cal. mgs.
The only combinations produced that saw combat were six or eight .50 cal. mg and 2 .50 cal. with the glass nose. Two aircraft were produced with the 75mm cannon for evaluation but never left the States.
wrongway