“Then why did the first ones require a B-29?”
Because it was the first one and was over-engineered by about 1000% due to the VAST sums of money spent to develop the nuke.
Disagree on the wildly over-engineered. Father worked both at Los Alamos and Hanford (where I am today) and didn’t ever suggest that was the case. The implosion-type devices received only minor changes in the next few versions.