Experimental prototype with 1,280 Merlin XX engine and a retractable tail-wheel, clipped wings, a redesigned front windscreen and a slightly longer fuselage. The Mk III also had extra panels which fully covered the undercarriage when retracted (as per the Spitfire prototype). The Mk III was to be a new better breed of Spitfire. In the event the easiest way to keep volume production of the Spitfire going without the difficulties of making major changes on the production line was to produce the Mk V instead. In any event production of the Merlin XX engine had to be used for the Hurricane, the powerful engine being the only way to keep the slower, but more easily produced Hawker fighter battle-worthy. The sole Mk III was used as a test aircraft and pioneered the use of the Merlin 61 engine.