Dennis was born in 1918 in Rochford, Essex to Braham Bernbaum and Fanny née Lipman.

Dennis was one of three brothers who served in the RAF and their father Braham served in WW1 in the Northamptonshire regiment. Dennis married Muriel Kerner in Edinburgh, her home town, on 25 November 1942. The certificate states that Dennis was serving in the RAF as a pilot officer and his original trade was as an investment company clerk.

Dennis was the pilot of Lancaster JB699 which departed from RAF Woolfox Lodge at 19.13 as part of Operation Gisela. They were part of a diversionary force of ninety-five aircraft supporting main force operations. At 00.40 hours an air raid RED was received and this was transmitted on Base D/F. At 01.25 hours JB699 was returning from its mission, and Flight Lieutenant Baum called up on radio and was immediately given the code word for a diversion and told to switch off his navigation lights. This was acknowledged, and about five minutes later a combat was seen to be taking place just north of the airfield. At 01.35 hours JB699 was shot down by an enemy intruder near Cottesmore airfield killing all the crew.

The crew members of JB699 were:
Pilot, Dennis Jacob Baum
MUG, F/O Kenneth Richard Brook DFC, 416165
Nav, P/O David Clifford Davies, 177181
W/Op Flight Sergeant Charles Edward Gardener, 422384, RNZAF
Sgt, Thomas Platt, 3011828, Rear Gunner
F/E, Sgt John Arthur William Smith, 1524154
A/B, Flight Sergeant Ronald Warne, 1622428