One museum display shows models of the various launch vehicles. All of them were derived from previous ballistic-missile designs. (In fact, the SpaceX Falcon 9 launch vehicle, not shown here, is the first U.S. launch vehicle not to be based on a missile.)
From left to right: Minuteman I (missile), Thor-Able (origin of today's Delta II), Thor-Ablestar, another Thor variant, Thor-Agena, Atlas-Agena, Atlas-Centaur, and one that I can't read.
Copyright 2011, Garrett Wollman. All rights reserved. Photograph taken 2011-02-26.