Just a quick note here,
The were 15 B-29's assigned to the 509th group. They were called Silverplate and designed to carry the A-Bombs. They could also carry regular bombs. They used to practice with something called a pumpkin bomb
Here is a link to the B-29 web site with great stories and other information.
http://b-29.org My uncle, TSgt Rod Arnold was the flight Engineer on the B-29 Bockscar, the second A-Bomb mission aircraft. The crew was replaced by the CO's crew (RHIP) and flew in the second chase aircraft (dropping instrument packages) The Great Artiste.
While at the Air force Museum I look in the archives for the aircraft forms and found his signature there.
His stories were allways interesting and full of adventure!