I think the part people are not prepared for is how tough this plane is going to be. think p47, f6f hellcat toughness and multiply that toughness by a billion and thats likely where the b29 will be.
the report from the p61 black widow emptying its full ammunition clip into an unmanned b29 to down it and still barely denting it makes me think a 109k4 will not even dent this with anything but a cockpit shot.
50 cals will likely be useless as well.
about the only thing with the firepower would be a 190a8 or ta152 and then just barely would they probably get one down out of the formation if they survived the defensive fire.