I reckon they used a drawing pin with pencils on three carefully measured lenths of string to get the outlines, then whoever hadn't been on the sauce the night before (ergo, steady hands) filled in the colour with a Windsor and Newton Number 24.
Oh, hang on, flaw in plan, METAL aeroplanes, can't get the drawing pin in... Back to the brain storming Smithers (well one could say that back in the Twenties, before all this anarcho-pinko PC nonsense...)