The trademark of the Spitfire, the Elliptical wing design, seen here at the Planes of Fame Air Show, Chino, CA.
Beverly Shenstone, the aerodynamicist on Mitchell's team, explained why that form was chosen:
" The elliptical wing was decided upon quite early on. Aerodynamically it was the best for our purpose because the induced drag, that caused in producing lift, was lowest when this shape was used: the ellipse was ... theoretically a perfection ... To reduce drag we wanted the lowest possible thickness-to-chord, consistent with the necessary strength. But near the root the wing had to be thick enough to accommodate the retracted undercarriages and the guns ... Mitchell was an intensely practical man... The ellipse was simply the shape that allowed us the thinnest possible wing with room inside to carry the necessary structure and the things we wanted to cram in. And it looked nice."
Canadian Aviation Blog