just because something was made to deal with something else, doesnt mean it could actually do it.
the bombing altitude of the b29 is well up in the high altitude 20k-35k range.
the Raiden lacked a supercharger. crippling it in high altitude flight. it could barely catch a b29 from dead 6 and you'l spend most likely 15-20 mins just catching up to it.. the only luck it ever had of catching one is if it made diving passes at them. so to catch a B29 in a Raiden you'd have to be in the air, know the b29's exact altitude, and its exact flight path. so you can make one screaming pass at it.
the Raiden was also a dog, no match what so ever for the mustang. so if the b29's have escort, throw your chances of knabbing one out the window.
i think the german planes are a better match of fighting off the b29 than japs.