The B-26 defensive guns, cover a much greater arc, IMO, than a Lancaster which has simular defensive gun positions. Also, they are actually pretty fast.
So often, people thinking that the B-26s do not have enough belly protection as the Lancaster does not, think they can approach safely from low 6 to the belly position.
However, if the alignment is anything less than perfect with the flightpath of the B-26, the interceptor will most likely face multiple .50s, since one of the gun either on port or starboard fuselage, will cover the firing arc.
So if to avoid that, an interceptors lines up perfectly from low-6 - then he'll face one less .50, but it will be a very easy aim for the rear gunner.
Now, that's against one B-26. If the B-26 is in a formation, the three planes cover a pretty huge area, even the low-6 positions - so practically there's no real 'safe' entry for an interceptor against a B-26 - the best chance is to fly a plane with big powerful guns, and have a massive alt/speed advantage, so the interceptor can damage the buff severly with a very quick burst on target.
In any plane that might require an actual 'aim' and a long 'burst' against a buff, it is most likely the plane will be shot down the moment he stops maneuvering, and lines up his shot.
The B-26, aside the B-17, has got the nastiest defensive system of them all.