R J Mitchell and the Supermarine design team wanted "the thinnest possible wing with sufficient room inside to carry the necessary structure and things we wanted to cram in" (aerodynamicist Beverley Shenstone in Spitfire: Pilots' Stories, Alfred Price).

The Hurricane's gun arrangement was slightly preferable for concentration of fire and ease of re-arming and maintenance, but the ammunition feed for the inner guns passed over the top of the outer guns; the Spitfire's wing was too thin for that, hence the pairs of ammunition boxes between each pair of guns.