I always suggest getting a DNA test, preferably with both guys, just to put the matter finally to rest and never to have to wonder again in your life. If it is not possible to tell one guy, test with the other. I would operate throughout the pregnancy on the assumption that guy B is the dad, but would test.
It really sounds like B is the dad. You say is there "any possible way" the baby is from guy A. Well, sure, counting out dates from early ultrasounds is very helpful but not foolproof. But from the dates alone, helped by the fact that you had an early ultrasound, it sounds like it is guy B's child.