As we all know in the OTL, the Boulton Paul Defiant was not a success as a combat plane.
In my humble opinion, I think there were two things that hurt the Defiant: 1) it needed a WAY more powerful engine and 2) four 0.303 inch machine guns in a turret was not very useful to attack other planes.
But what if the Defiant was able to use the way more powerful Rolls-Royce Merlin 61 engine rated at 1,565 bhp (with a four-bladed propeller) and replace the four 0.303 inch machine guns with two 20 mm Hispano cannon? The result would be a way faster plane that could have pioneered attacking bombers from below (akin to what the Luftwaffe did with the Schräge Musik installation on the Bf 110 and Ju 88C fighters) with some pretty nasty results.