This is what I had in mind:
(Quote from Wiki. I've also read similar reports elsewhere).
Initially, Luftwaffe fighters suffered losses when "bouncing" flights of Defiants from the rear, apparently mistaking them for Hurricanes.[12] The German pilots were unaware of the Defiant's rear-firing armament and encountered concentrated defensive fire.
With a change in Luftwaffe tactics, opposing fighters were able to outmanoeuvre the Defiant and attack it from below or dead ahead, where the turret offered no defence. Defiant losses quickly mounted, particularly among the gunners, who were often unable to leave stricken aircraft. The additional weight of the turret and the second crewman plus the aerodynamic drag gave the Defiant a lower performance than conventional single-seat fighter aircraft.[13]
http://en.wikipedia.org/wiki/Boulton_Paul_Defiant
Best Regards,
MB_Avro