UA's policy states that it will not check across tickets outside *A. Thus, won't check across separate tickets to AA. The policy does not refer to PNR's. UA & AA do have an interline baggage agreement so it comes down to getting lucky and finding the right agent. You can't count on it and need to be prepared to fetch your bags at IAD, check them in with BA and then head back through the appropriate checkpoint to the B concourse for your AA flight.

You'll need to go to baggage claim and re-check your bags if:
....

• You’re connecting to an itinerary booked on a separate ticket that doesn't include a Star Alliance™ partner airline