Harryhausen has mentioned at different times he had ideas for some of his films that got dropped prior to production, and how he would tuck those concepts away in hopes of finding them homes in future projects.  While looking through some of his artwork, it would seem that recycling mindset was applied to his illustrations as well.  Compare these two shots for unrealized projects, WAR OF THE WORLDS and THE ELEMENTALS.  (Appologies on the WAR OF THE WORLDS pic...couldn't find a good copy and had to enlarge it.)  Both show scenes that would have given us a cockpit point-of-view of the action.  Except for what's seen through the window, the two pieces are identical.  Was Harryhausen able to replicate all those details of the cockpit and pilot that precisely in a second drawing, or did he simply draw over part of his original illustration to replace monsters with Martians (or vice-versa)?