The B-57 also had 2X the bomb load of the XB-51 (B-57 = 8,000 pounds, XB-51 = 4,000 lb both at useful range), along with 2.5x the loiter time at twice the range from base which comes in handy if you are talking about a bomber.
Strafing isn't a bomber's game, even a light bomber, its a ground...