 Bowen led a series of lectures at Bentley Priory , on the theory of radar guided night interception and concluded that the fighter would require a speed advantage of 20 to 25 % over its target . The main Luftwaffe bombers — the Junkers Ju 88 , Dornier Do 17Z , and Heinkel He 111 — were capable of flying at about 250 miles per hour ( 400 km / h ) , at least with a medium load . This implied a fighter would need to fly at at least 300 miles per hour ( 480 km / h ) and the Blenheim , fully loaded , was capable of only 280 miles per hour ( 450 km / h ) . Bowen 's concerns over the poor speed of the Blenheim were proved right in combat . 
