The Mk. IX replaced the Mk. V in June 1942 and on September 12, 1942 a Mk. IX brought down a Ju-86R flying at 43,000 feet. Another major achievement of the Mk. IX took place on October 5, 1944 when 401 Squadron became the first allied aircraft to shoot down an Me-262 jet. Even with the arrival of the Griffon powered Mk. XIV the Mk. IX continued in service until the end of WWII. There were 5,665 Mk. IX Spitfires and its variants produced.

W. Duncan-Smith began his military service with the 7 Operational Training Unit (OTU) before being posted to the No. 611 Squadron and in June 1941 received the DFC. Eventually Duncan-Smith took charge of the 324 Wing in the Mediterranean. Duncan-Smith flew Spitfire IX JK650 'DS' as a Luqa Wing OC, and later flew Spitfire IX MH884 'DS' from Corsica to Ravenna as Group Capt and OC of 324 Wing 1944-45. He is credited with 17 aircraft shot down, 2 shared shot down, 6 probables, 2 shared probables and 8 damaged in aerial combat.