4 foreward 20mm, plus 4 50's in a dorsal barbette (P-61B-15 and later), plus 4 hardpoints able to hold drop tanks or up to 1600lb bombs each (yes, FOUR 1600lbs bombs) (B-10 or B-5 and later, cant remember exactly)
Contrary to popular belief, the barbette was functional in the B-15 varient and later, after some of the aerodynamic problems were fixed. It covered the entire upper hemisphere of the aircraft, front to back, and 90 degrees up, directed by a gunner in front, or the radio opperator in back. The only restrictions to the barbette's aim was the two vertical stabilizers in back and the prop discs in front. It wouldn't depress enough to create a hazard of hitting the wings or horizontal stab.
For modelling how the barbette works, methinks they could make it a turret with guns but unmanable, and make the radio opperator (back gunner) and front gunner's positions manable, have the turret director sights, but no guns. Use the 'fire all guns' control to get the barbett to shoot. Now how they might make the barbett also fire forward under pilot control? No idea.
As mentioned, these things were used in the day as ground attackers and such. Note the big bombs they can carry mentioned above just for such a role.