” ... “The Whole 9 Yards” also from WWII. The machine-gun belts were 27 feet long, so when the gunner fired the entire belt, he gave them the whole 9 yards.”
Whatever the origins, this has become an internet urban legend. The other attributions (kilts, baseball) probably have a more credible claim as to origin, as it could not possibly have originated from fighter aircraft in the Second World War.
By the late 1930s, all (allied) aircraft guns (fixed or flexible) used disintegrating metal-link belts, which do not have any fixed or standard length. They were pieced together round by round and link by link, to the exact size required to fill the ammunition boxes of any particular airplane and application (some aircraft mounted both fixed and flexible guns - B-25H, A-24, Grumman Avenger, A-26, B-26 are some).
No two different makes/models of aircraft were equipped with the same ammunition boxes.
US and British fighters (and nearly all other aircraft flown by those two countries) were armed with Browning guns during WWII. “Standard” fabric belts were still used at that time for ground guns and they held 250 rounds in the 30 cal version. Just a minute or two with a ruler can tell you that such a belt will never be nine yards long.
If there is any chance that the slang originated in ordnance circles, it likely dates to the First World War. Canvas belts for the British Vickers gun held 250 rounds when standard-built; the cartridge pockets are spaced quite differently than those in a Browning belt. Length is remarakbly close to 27 feet.
The Maxim, the other widely used gun of the day (very extensively by Germany and Russia/USSR in both World Wars), used a belt of very similar configuration and size.
Threads like these are what define the greatness of Free Republic.