Designer of Spitfire was, unfortunately, dead before ww2; granted, it was not only Mitchell that single-handedly designed the Spitfire. Designer of the P-40 left the company before 1942.
MTT have had several design teams on the payrole, so the success is not a surprise. They were coming out with designs ranging from trainers to 4-engined stuff. Granted, MTT managed to produce several flops, too. Supermarine was a small company, part of Vickers.