Ok, Saxman is going to flip but here goes anyway:
"WEDNESDAY, 21 MARCH 1945
AMERICAN THEATER
ZONE OF INTERIOR
(Fourth Air Force): A P-63 from Walla Walla AAFld, Washington, intercepts a Japanese balloon near Redmond, Washington, and, after a chase that includes 2 refueling stops, shoots it down near Reno, Nevada."
Source:
http://www.usaaf.net/chron/45/mar45.htm, i.e. the online history of the USAAC in WWII
Sooooo, here's an air-to-air "kill" of a Japanese instrument of war, by a US Pilot no less, over US airspace, well documented.
How about that?
Here's a little more detail on the incident, and more info in general on the Japanese balloon "offensive":
http://aviationtrivia.blogspot.com/2010/12/japanese-balloon-offensive-of-1944-1945.html