LeMay had the B-29s fly lower, at night, so more bombs could be carried. At those lower altitudes, could carry 10,000 pounds, 1840 nautical miles. Targets shorter than that, more bombs could be carried.
Cruising at 20,000 feet before climbing to over 30,000ft like the Atomic missions, had 1600 nautical mile range.
You don't want to drop a 15kt atomic bomb from 20,000 feet, not enough time to clear the target area.
From 34,000 feet Enola Gay had 44 seconds from release to detonation to get as far away as possible. It was bomb release, and then hard turn. IIRC had a nine mile slant range, and the aircraft still experienced severe buffeting