Steven F. Udvar-Hazy Center near Washington Dulles International Airport.
It is the companion facility to the Museum on the National Mall. The building opened in December, 2003, and provides enough space for the Smithsonian to display the thousands of aviation and space artifacts that cannot be exhibited on the National Mall. The two sites together showcase the largest collection of aviation and space artifacts in the world.
I have been there several times. The exhibits continue to roll in. The last time I was there I discovered a new aviation related firearms exhibit!
If you every get a chance stop by and see them. This history, these artifacts are not to be seen anywhere else. Even books and websites are thin on these artifacts.
--Air and Space and Guns!