The XB-42 is an interesting aircraft. Basically it would have been the second generation of Mosquito style aircraft - fast unarmed bombers.
It had .50 cal guns directed fore and aft, but not in turrets.
The pusher configuration was partially the reason for this - as it allowed the airframe to be much cleaner.
This is certainly true in that there were no engines on the leading edges of the wings. These same two engines could have driven similar contra-rotating propellers in the nose with no less of a clean airframe. This would, however, have led to greater drag caused by the uneven airflow over the wings and fuselage due to the propwash.
One obvious advantage of having the props at the rear was that the bombardier had a clear, unobstructed view from the glass nose. One obvious disadvantage was that it made bail-out much more dangerous for the crew.
There is one issue that I wonder about. A pusher propeller at the extreme aft end of the fuselage causes a loss of stability over those located further forward or over tractor configurations. Since the XB-42 was a bomber, and bombers require good stability in the bomb run, was this a drawback of the design? Perhaps it was not of a magnitude great enough to have caused a problem.
The Mixmaster is quite an ugly looking aircraft. I believe it was the first to use Douglas' double bubble cockpit canopy style for bombers.
Do you really think it was ugly? I kind of like it. I do, however, think that the later single canopy looked a lot better than the original "double-bubble" type.