I should, perhaps, have been clearer: I meant "use", not "building". That, per Alperowitz, was a joint decision by Byrnes & Truman.
The decision to whether or not to use the Bomb, and HOW, was chaired by Byrnes. But he wasn't the Lord & Master of that committee, just one strong voice among many. Numerous people made suggestions, each in turn dismissed as unworkable or liable to boomerang on the Allies.
Frex, telling the Japanese the target after expending the Tall Boy on a remote Japanese island under full Japanese view. Then tell them to surrender, and if they didn't the Allies would hit a specific city at a specific time and date. The realization came to the committee (and I will say it was Byrnes who said this) that if they did so, the Japanese would have their entire remaing air forces, including suicide attackers, waiting for the Silverplate bomber. And they'd evacuate the city, replacing the population with PoWs.
I REALLY hate to invoke the words of America's Worst President Ever (minus Buchanan), but it was TRUMAN who was "The Decider". The fact that Byrnes felt so strongly that HE should have had Truman's job as POTUS notwithstanding.
Which explains why that notorious dog, the P-38, had two of them.
It was a dog, over Europe. In the Pacific, that was another matter. At the latitudes and altitudes it operated in against Japan it represented the first true answer to the Zero, provided the USAAC pilots weren't too hotshot to ignore their rules of tactical engagement (bounce 'em from above, blast 'em, don't dogfight, and run like hell).
Tell you what: We both have Skippy the Alien Space Bat do his thing. We are both now trained and qualified P-38 fighter pilots, and its WWII. One of us has to go to Europe, the other the Pacific. Which one of us goes to Europe? Hint: It ain't me!