109 cant do XXXX anymore.

[Crumpp]

One could deduce that physiology was a major player in anything the USAAF did.

Yes.

Carrying bombs was not the limit, a human beings ability to survive and function at altitude was the limiting factor.

That being said, the B-17 series approached the limit of what was practical for a high altitude bomber of the day. Prolonged operational campaign at 20,000 feet was a milestone given the technology.