From a web site I read every day

‘The Brits Almost – Almost – Got to Drop the Bomb, But …’

“The British Black Lancaster bomber might have been designated to carry the bomb to Hiroshima on 6th August 1945, but it was too slow and did not have the weight load capacity that our silver B-29 bombers had.”

https://valorguardians.com/blog/?p=103388

My response:

“So the Lancaster ‘might’ have carried the American bombs, but it was too slow and didn’t have the load capacity of a B-29. And why should the US have considered the Lancaster to carry the US bomb? Just a guess, but I would say the Brits would have found a reason not to drop the bomb.”