German air supremacy at night was also now under threat. British night-fighter operations out over the Channel were proving highly successful. This was not immediately apparent. The Bristol Blenheim F.1 was undergunned, with just four .303 in (7.7 mm) machine guns which struggled to down the Do 17, Ju 88, or Heinkel He 111. Moreover, the Blenheim struggled to reach the speed of the German bombers. Added to the fact an interception relied on visual sighting, a kill was most elusive even in the conditions of a moonlit sky.