On the wing?
(http://www.caphistory.org/galleries/Airplanes/large/L-16%20Aeronca_DS_side%20view.jpg)
If you mean this:
(http://img17.imageshack.us/img17/46/usaf.png)
Then download the attached TTF.
Awesome, THANKS!
It didn't start out as a typeface, pre se.
The spec for making the letters and numbers was based strictly on dimensions and proportions.
For a given height, the character shall be x percent of that height, and the lines shall be y percent. The corners shall be 45 degrees, with the hypotenuse being z length, on the outside, and proportional on the inside. There were also specs for the wide and skinny letters, related to standard width.
The spec allowed layout on the side of a plane with just a ruler, scissors, and tape.
Somewhere out there on the intertubes are the various services' manuals on doing insignia painting.
[edit]
FWIW, this is close, but not totally correct. The lines are too narrow, and it doesn't kern correctly.
Close enough for government work, as they say (which is actually the opposite of its original meaning).
Looks great where I'm using it.
I have found this search helpful in finding different fonts I have wanted to use.
http://www.fonts.com/FindFonts/SearchBySight.htm (http://www.fonts.com/FindFonts/SearchBySight.htm)
This is the mil-spec for A/C markings"
http://www.scribd.com/1Anonymouspatriotusa/d/12930959-Navy-Aircraft-Colors-Markings (http://www.scribd.com/1Anonymouspatriotusa/d/12930959-Navy-Aircraft-Colors-Markings)
Pages 51 and 52 have the lettering details.
Air Force aircraft markings are contained in TO 1-1-4, several versions of which can be downloaded from here: http://www.usaf-sig.org/index.php/downloads/category/7-color-markings-related (http://www.usaf-sig.org/index.php/downloads/category/7-color-markings-related)
It's interesting that although similar, the USAF and USN letters aren't the same. The USAF also gives its lettering instructions in a much simpler format than the Navy. An example of the USAF lettering is on page A-8 (pdf page 20) of part 2 of the 1985 TO. The markings for an O-1 and T-41 are given in 1985 part 5.
The history of AAF/USAF aircraft markings is available from the Air Force Historical Studies Office: http://www.afhso.af.mil/questions/topic.asp?id=1960 (http://www.afhso.af.mil/questions/topic.asp?id=1960)
Mike