I just removed my earlier reply which was largely obsolete.
On the Dulles web site
http://www.mwaa.com/dulles/index.htm
http://www.mwaa.com/d2/index.htm
you will find
"In 1996, a second Customs and Immigration facility was opened by United Airlines in Concourse C, further streamlining service for international travelers making domestic connections in the U.S."
You should have no trouble with the connection (presuming that facility is still in operation).