Why was the Boulton & Paul Defiant not a very good fighter aircraft?
Briefly, it lacked the necessary speed which was important for fighters at the time. The gun turret mounted behind the cockpit was heavy and added drag to the design, and anyway had already been proved to be flawed idea by the end of the First World War, during which all the most successful fighters had fixed forward firing guns, the aiming done by pointing the whole plane rather than rotating a turret. There were plenty of similarly impractical ideas in military design during the 1930s, such as multi-turreted tanks and the like. As soon as they were thrown into the crucible of combat in the Second World War their shortcomings were quickly revealed. Apparently the Defiant had some limited success when mistaken for a Hurricane by German pilots and attacked from behind, but I am not sure if this wasn’t just an apocryphal story.
