JohnB wrote:
Bdthe B-17 and B-29 used 24ST,the B-50 switched to 75ST, it's one of the major engineering differences between it and the '29.
Are you talking the skins, the extrusions, forgings, or what? I'm not sure your blanket statement is accurate, especially considering production changes.
From Wikipedia, "The first 7075 was developed in secret by a Japanese company, Sumitomo Metal, in 1935, but introduced by Alcoa in 1943 and was standardized for aerospace use in 1945. 7075 was eventually used for airframe production in the Imperial Japanese Navy."
https://en.wikipedia.org/wiki/7075_aluminium_alloy