do not leave IAD with a 3.5 hour layover.

but, you can bar hop around IAD. start at gordon beirsch at concourse D, and then make your way over to B and stop at harry's tap room, and finish yourself off at dominion brewing company! easy way to burn a few hours.

if you leave IAD to go over to udvar, the odds of you not making your flight are pretty high. security can be terrible, and transportation to/from udvar can take some time. so i would stay in the airport.