This is not coming straight from the horse’s mouth, but close enough. I recently took a tour of a WWII bomber , not a fighter, in which a veteran who fought in one of them showed us the on-board machine guns used against enemy fighters. He told us that they had nine yard long ammunition chains and that their gunners originated “the whole nine yards” expression. This bombers is on display at the Palm Springs Air Museum, by the way.
Not close enough. I’m old. I mis-remember things. I tell you what I’ve been told all my life. We all do.
There are NO print cites for the expression until 1966. I have no doubt that it is military in origin. But not as far back as WWII.
Museum tour guides are perhaps the biggest spreaders of urban legends today. I always thought Paul Harvey or Dear Abby were. But tour guides are so much more ubiquitous.