The E-1 carried two 7.92 mm MG 17s above the engine and two more in the wings. The E-1B became the first operational Bf 109 fighter bomber. These were fitted with either one central bomb rack, carrying one 250 kg bomb, or two under-wing bomb racks, each carrying a 50 kg bomb. The E-3 was armed with the two MG 17s above the engine and one MG FF cannon in each wing. The E-3 was replaced by the E-4 (with many airframes being upgraded to E-4 standards starting at the beginning of the Battle of Britain) which was different in some small details, most notably by using the modified 20 mm MG-FF/M wing cannon and having improved head armour for the pilot.