Warbird Information Exchange

Has anyone ever ran across an OFFICIAL reason why, during WWII, the numbered Air Forces went from 1st to 15th, then to the 20th ?
The 16th, 17th, 18th, and 19th Air Forces all came into being post WWII, during Korea of just thereafter, but why were those numbers skipped when assigning numbers to Air Forces during WWII ?

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SHAEF1944
American Veterans Museum

My memory isn't quite what it used to be, but I seem to remember reading in an official document that General Arnold "reserved" the 20th Air Force designator for the Very Heavy Bomber effort. The four unused numbers were intended to be assigned to units that were never constituted. I'll try to track down where I read this but I doubt I can find it now--it's been a few years since I stumbled onto the information.

Scott