F/O Allen has taken part in all combats with Flt.Lt. R.H.A. Lee, following his section leader with great loyalty. In May 1940, he took part in shooting down a Junkers, and the next day another enemy aircraft of the same type. On the latter day, after his aircraft had been severely damaged by anti-aircraft fire, he did not hesitate to attack with vigour and determination a Junkers 89 over enemy occupied territory and shot it down.

On 18.05.1940 Hurricane P2701 on a mission in the Lille-Seclin area was shot down in combat with Bf110s of I./ZG26 during offensive patrol between Le Cateau and Cambrai 7.30 a.m. Flying Officer D.H. Allen missing. Aircraft a write-off. His body was rescued from the wreckage by locals before being buried in an unnamed grave in the village of Pox-de-Nord near the Belgian border, resulting in a mentioning on the Runnymede Memorial as missing, presumed killed.