Dulles-Washington is a weird airport - or at least it was the last time I went through it on the way to Orlando. Granted that was about 4/5 years ago, so it may well have changed, but if it is the same, then 1hr 40mins would actually be quite do-able.
On disembarking the plane, there were two separate signs to immigration/customs - one for customers who were staying in Washington and one for people who had other flights to catch to elsewhere - and they both went in different directions. These were not like the Flight Connections signs at Heathrow/Gatwick where they both point in the same direction as the other immigration signs and only split off after you're through that area - these went to two completely separate immigration areas in different parts of the airport.
Because of this, when we got to "our" area, it turned out that maybe three quarters of the people on our flight (and I guess any other flights arriving at the same time) must have been staying in Washington, and so the queues we were in were really quite short and we were able to get through quite quickly. And once we exited customs, we followed the corridor and it basically dumped us out right in the middle of the concourse no more than 50 yards from the gate for our Orlando flight - can't guarantee you'll be that lucky though!
Also, this was flying with United, so it may just have been a feature of the terminals that they landed at / took off from.