I have flown to Washington with ds and dh since he was 4 months old about twice a year, and am going again in 2 weeks time, just with ds.
And I would always go Virgin, as I think they are fab - very helpful, nice new baby cots, although ds never spent more than a few minutes at a time in them, great support in the air.
They never even raised an eyebrow at continous bfing, and when older, happy to have ds roaming the ailes and saying hello to people (when they are not doing a food/drink service), helpful with heating milk, provide nice kids snacks and drinks,and just generally nice and cheerful.
Just some words of caution about Dulles - as far as I know, no airline can get your buggy to the gate, they are all sent to the luggage area, so it is a good idea to take a baby sling of some kind, as you can be stuck in immigration area for ages just queuing.
If you are bold and lo is young, ask ground crew to take you through the fast lane, due to fact that buggy was not at gate. I did not plan it, but worked for me when ds was really small.