At least one news story I read (sorry, forgot the source -- I've read so much in the last week -- but it was a credible source, eg NYT or Wash Post) indicated that fighters were scrambled: F-15s from Hyannis, on Cape Cod, in reaction to the hijacks from Boston. They apparently didn't reach New York until a few minutes after the second crash. F-16s were also scrambled from Norfolk, Virginia, in response to the AA hijacking from Dulles. Apparently they went supersonic most of the way, but still got to DC after the Pentagon had been hit. But presumably they would have been in position to intercept the UA 757 had it in fact reached the city.