I only know the transfer process to UA and there pax are separated into arriving and connecting pax and are processed separately. Might or might not be the same for AA.
12:40 might be ok, certainly better than between 3 and 4.
One important tip: most likely you will be transported on a mobile lounge (moon buggy) from Terminal B to immigration facility. Try to stay at the very front of the vehicle, so that you are first off when arriving at immigration facility!