 With October 's deliveries , the Air Ministry began plans for a production AI Mk . II . This differed largely by the addition of a new timebase system , which it was hoped would reduce the minimum range to a very useful 400 feet ( 120 m ) . When the new units were installed , it was found the minimum range had increased to 1000 feet . This problem was traced to unexpectedly high capacitance in the tubes , and with further work they were only able to return to the Mk . I 's 800 feet . Blenheims from a number of squadrons were fitted with the Mk . II , with three aircraft each being allotted to No. 23 , 25 , 29 , 219 , 600 and 604 Squadrons in May 1940 . 
