That may not be as far-fetched as it sounds, in fact it has already been done!
Scientists in the UK designed a small UAV using CAD-CAM software, printed it out with a 3D printer, assembled it and flew it all in a five day period. This is a sophisticated design, too, with a geodesic structure.
3D printing is a relatively new technology that works similarly to a paper printer, except that the 3D printer builds up layers of plastics to form a three dimensional shape.
Now it is true that this is only a small aircraft with a wingspan just under 5 feet, but you have to start somewhere. It wouldn't take a lot to scale this up to man-carrying size and essentially print your own kitplane. The main advantage here is the speed at which this can be done.