S/Ldr McMullen joined 54 Squadron in September 1939. Prior to the Battle of Britain Desmond had claimed an Me110 destroyed over Dunkirk, along with two probable Me109s.
During the battle he was credited with 13 destroyed. After a period with 141 (Defiant) Squadron, Desmond was posted as Squadron Leader to 64 Squadron.
Desmond was posted to 65 Squadron and then 324 Wing in North Africa. He remained in the RAF after the war and retired in 1957 with the rank of Wing Commander.
W/Cdr McMullen passed away on 1st July 1985, aged 67.