I think this is your aircraft. Curtiss XP-71
http://en.wikipedia.org/wiki/Curtiss_XP-71
Designed as a very heavy fighter (larger than a B-25!) designed to operate over Europe if England was ever conquered. Has one 75mm and two 37mm cannons.
Not sure how this would ever be useful. An aircraft of that size during WWII wouldn't have the agility to defend bombers. It'd be one hell of an attack aircraft mind you... over 400mph speed.