The problem with the A-bomb was it's complexity. It was not a big can of explosives with a firing mechanism like most bombs. Nothing like it had ever been made and it's internal workings were extremely complex and not all that robust. It probably would not have withstood hitting anything let alone hitting the earth after falling 30,000 feet or more. Since we only had three of these bombs we did everything possible to make certain they would all work. I recall the bombs being triggered at 1500 feet. I don't know if that number is correct but if the plane was at it's maximum altitiude of ~40,000 feet it would not have had enough time to get away if the bomb was triggered at almost 20,000 feet.