The B-57B introduced several other major changes. In terms of armaments, a total of four hardpoints attached to the outer wing panels were installed on this variant, capable of mounting external bombs and rockets. The B-57B was also furnished with a gun armament for strafing attacks. The first 90 aircraft to be produced were equipped with a total of eight 0.50 cal (12.7 mm) Browning machine guns installed within the wings; on later aircraft these were replaced by an arrangement of four 20 mm M39 cannon. A new rotary bomb door was adopted on the B-57B, which had originally been designed for the XB-51, replacing the hinged doors; this proved to be faster to open/close and reduced buffeting, which in turn improved accuracy and allowed for faster attack speeds. The redesigned bomb bay also enabled faster turnaround times on the ground.