Neil,
There weren't many B-29's in the OD over gray finish. My guess is those airplanes did not have the rubber inflatable deicer boots installed yet. Those are glued on with screws in a few places for additional reinforcement. Glue doesn't stick well to paint, hence the bare metal:
http://en.wikipedia.org/wiki/File:Olive-drab_painted_B-29_superfotress.jpg
If I remember correctly, the domes on the upper fuselage are the sighting stations for the two upper gunners. The turrets were remotely operated:
"The B-29's revolutionary Central Fire Control system included four remotely controlled turrets armed with two .50 Browning M2 machine guns each.[N 2] All weapons were aimed electronically from five sighting stations located in the nose and tail positions and three Perspex blisters in the central fuselage.[N 3] Five General Electric analog computers (one dedicated to each sight) increased the weapons' accuracy by compensating for factors such as airspeed, lead, gravity, temperature and humidity. The computers also allowed a single gunner to operate two or more turrets (including tail guns) simultaneously. The gunner in the upper position acted as fire control officer, managing the distribution of turrets among the other gunners during combat."
The props are Hamilton Standard Hydromatics. The domes are normally bare metal, but were often painted in the field. Most of the ones I've seen are either bare metal or black, but you'd have to check your references if you're doing a specific airplane. You'd be safe to leave them in silver, but notice the ones in the above photo are black.
A good friend of mine flew the CAF's B-29 "Fifi" a while back. If you have any more questions, I can ask him for you.