There was no
Little Boy test as they only had enough U235 for the single weapon.
The earlier configurations for the 'gun-type' weapons, the uranium
Little Boy and plutonium
Thin Man, were 17 feet long and at one point carriage by a Lancaster was proposed. One B-29 was modified
to have a single 33' long bomb-bay, this was referred to as the 'Pullman' aircraft. However it was discovered that the projectile speed requirement for U235 wasn't as high as thought, so the weapon was able to be shortened to six feet, allowing carriage in the standard B-29 bays.
The casing design for the gun-type plutonium weapon was built and tested by air-drops from the
Pullman B-29, aerodynamic performance was poor and the realities of plutonium fission meant that
they had to go the implosion route.
Thin Man casings with
Fat Man casings in the background.
The single point bomb carriage/release mechanism was the same that had been developed
in the UK for Tallboy and Grand Slam.