Gruss Gott Gunther!
As RB stated, airbrushes were around during WWI, and there were some a/c that had paint airbrushed. (Some of the camo on some of the Nie.11's and 16's comes immediately to mind. I know there were others.) However, it certainly was not all. Best to closely look at images, etc. of which crate you're interested in replicating, etc.
Warren