Attention aircraft/spacecraft concept artists looking for inspiration! Check out RetroMechanix.com, which is devoted to "Yesterday's Wings of Tomorrow." It features nearly 800 high resolution images of little known vintage aircraft proposals and prototypes; it's a great resource if you are looking for an unusual configuration or want to add a retro influence to your design. Some examples include:
The Douglas Model 1875 Aircraft Nuclear Propulsion Logistic Carrier (1955):
The McDonnell Douglas Model 226-458 Quiet Attack Aircraft, a stealth proposal from 1974:
The Douglas Model 1186-C Long Range Special Attack Aircraft:
The site also features several examples of 1950's graphic design, which again may be useful if you want to add a retro look to the presentation of your concept vehicle:
New articles and graphics are added several times per week, so check back frequently if you are interested in this type of material.