It really depends on how long it takes to get through immigration. We flew from Heathrow to Philadelphia then had an hour to catch our connection to Jacksonville which we did with time to spare. The only time I went from Heathrow to Dulles (with an hour and a half to catch the connection) it was a nightmare! It took three hours to clear immigration, and security, then two hours waiting in line at 'customer care' to be re-booked to Jacksonville. There were no more flights that day so I spent a cold, uncomfortable night on a seat in Dulles airport until the following morning. If I were you, I would try to find a flight with a bit more time to play with. Good luck! 