Mk Ia
Engine: Merlin II, Merlin III
Wing: Standard (A)
Armament: Eight .303 machine guns
Mk Ib
Engine: Merlin III
Wing: B
Armament: Two 20mm cannon, later two 20mm cannon and four .303 machine guns
Notes: The prototype's skid was replaced with a tailwheel, the wing's skinning was different and its torsion box strengthened. The Mark I's ailerons were subsequently re-skinned with metal, to prevent their jamming in high-speed dives.