Jon,
I print a lot of my own decals, and so far haven't found any basic computer fonts that match the WWII USAAC fonts of the era. Some come close, but you'd need to screen most of them for suitability. There was a company a few years ago that sold "military style" fonts, but I dont know if they are still in business. I have both MS Word and Powerpoint fonts, but again, haven't found any perfect matches. And printing a yellow tail number for laying over a green paint scheme is almost impossible, unless you are using a laser printer. On the other hand Microscale decals has an entire selection of USAAC WWII style fonts in various colors that would do nicely. There are other decal making techniques you can also use-- applying dry transfers to decal paper, or lightly printing and hand lettering on decal paper with enamel or acrylic inks (if you have steady hands) and then coating with decal solution is another method. By the way-- if you are printing with a bubble jet printer, make sure you flat coat the paper first--the ink won't thin and run off that way, and depending on your printer, you can run the printed decals through several times for a deeper color (except for yellow--which just doesn's deepen much and becomes translucent-- for those I usually carefully hand paint over the top). You didn't mention what scale or the color of the tail numbers (I'm assuming yellow).
VR, Russ