 The arrival of the Mk . IV in July 1940 improved matters , but it was the delivery of the Bristol Beaufighter starting in August that produced a truly effective system . The Beaufighter had considerably more powerful engines , speed that allowed it to catch its targets , and a powerful gun pack of four 20 mm cannon that could easily destroy a bomber in a single pass . Squadron use began in October , and its first victory came soon after on 19 / 20 November when a Beaufighter IF of No. 604 Squadron destroyed a Ju 88A @-@ 5 near Chichester , very close to the first success of the Mk . III . 
