My 1st visit to the facility at Dulles was about 4 yrs ago, there seemed like a lot of planes, but I could tell from the amount of space remaining there were going to be more. Well I've been back 2 more times since then and there are lots more than I ever thought would fit, and they still bring in new ones.
The DO-335 Arrow (push-me / pulla- you) is amazing large for a fighter. You get an appreciation for how big a B-29 is when you see the Enola Gay how many planes they park around / under it. There is the original Boeing 707-80, I think they should have hung it inverted in honor of the famous "Barrel Roll". So much to see, make sure you have spare batteries and media for your camera.
The space wing off the back of the main gallery is also amazing. They have the Enterprise Shuttle that did the drop test / atmospheric flight / landing tests before they ever launched one into orbit. When I was there after the Discovery tragedy, they had some wing leading edge panel removed, later when I went back they were back on- but you could see where they patched the damage from the impact testing they were used for in the investigation.
Compare the Shuttle to the Apollo capsule and you have to admire the guys who would stuff themselves into something the size a VW bug and go for a 1/2 million mile cruise to the moon and back.
Or look at a Gemini capsule and try to imagine spending almost 2 week in a Miata orbiting the earth. It has been reported that the astronauts smelled so bad after they were picked up the helo crews were tempted to dunk them in the ocean for a quick rinse!
Oh yeah - and hanging from the ceiling is the Mustang that I will paint my modified Twiststang to look like- there are enough shiny silver and camo painted ones around.