Victor Davis Hanson has a couple of shorts on YouTube discussing this. He indicates that we had 2,000 additional B-29s that would have been delivered by the end of 1945, and several thousand other 4-engine bombers laying idle on airfields in England. Curtis LeMay had plans to up our bombing raids from 4 or 5 per week to 2 per day. He would have burned down every city in Japan within six months, and probably killed at least 2 million Japanese. That’s without blockading Japan from all food and energy imports, and before an invasion.
The atomic bombings were horrible - but worse could have been done, MUCH worse.
Most people who go on about “the evil A-bombs at Hiroshima and Nagasaki” have no clue whatsoever about LeMay’s bombardment campaign, and how much more effective it was about to be due to A) much closer basing on Okinawa and B) more aircraft with massive B-29 production and the relocation of the 8th Air Force, 15th Air Force and similar British units from Europe.
As a side note - the only armaments development effort bigger/more expensive than the Manhattan Project was the one that resulted in the B-29! Building a bomber big enough to deliver a useful bomb load across Pacific distances was non-trivial.