Mighty Aphrodite
Here's another alternative to consider.
It would have to be modified; it would hardly do to have even a minimal flight crew bailing out over a Japanese city about to get nuked! I suppose it would be possible to have a crew handle the takeoff, then bail out just offshore of their base, with boats standing by to recover them from the Pacific. After that the plane is on autopilot, nudged by remote control from escort planes to stay on course. Upon approach to target the final autopilot program is engaged; this holds the plane on a steady heading at the optimal airburst altitude and arms the bomb. The escort planes veer off and track it from a great distance.
Should Japanese fighters or AA disable the plane, either sensing devices aboard the drone plane or remote command from the observing escort planes triggers the bomb early and that way it does some damage to the enemy anyway.
As far as range is concerned, the drone is only going half of the round trip from base to site and back, and its only payload is the 5 tonne or so "device." I believe there were not only issues with sheer weight with the first-generation A-bombs but also their bulk; it took a B-29 to merely accommodate the bomb. But if we aren't actually dropping the bomb but merely detonating it inside an expendable airplane we have more flexibility.
Clearly a drone plane with simple WWII technology (as sophisticated as they could manage then, but simple compared to what we can do today) is not going to be able to do any maneuvering for its self-defense.
Perhaps it would make sense to purpose-design the drone plane instead of recycling an obsolete old bomber airframe. Then for the Enola Gay alternate mission, I'd consider a biplane, more of a sesquiplane really. The airframe holds the bomb, autopilot/remote control gear and fuel for the final sprint to target, and it's a high-powered trimotor or so, with 2 to 5 radial engines on the nose and lower wing--the lower wing being small and sleek. The upper wing contains fuel for cruise and is much bigger. The plane cruises toward target rather slowly using just one or a few of its engines (after takeoff on all engines).
It's escorted to target of course; we might not want to make the upper wing too big or the flight too slow so that reasonably capable escorts would not have to keep zigzagging around it and would have enough fuel left to get home!
When approaching target and armed for its strike, the upper wing is blown off and all engines engage at full power for a run up to very high speed, as fast as a WWII prop plane could possibly go. We rely on speed, and the fact that this minimal drone plane is a relatively small and low-visibility target, for it to evade defenses and reach the pre-set optimal detonation zone. Or again if it gets hit early, the bomb is set off early.