It is too so. The length of a .50 ammo belt for American fighters in WWII was 27 feet, or 9 yards. This wasn't just the spec for Mustangs, but also Thunderbolts, Corsairs, etc. The length was standardized and fighters were designed to take this length of ammo belt. Giving somebody "the whole nine yards" meant lining them up in your sights and pulling the trigger until the guns all went "click."
I didn't know that... cool.
You're absolutely right. My grandfather, a WW II U.S. Army Air Corps. Veteran (Pacific Theatre, 42-45), told me this years ago when I made some off-handed comment about the "whole nine yards."