The Americans preferred bombing by daylight.
Their reasoning was simple: Industrial targets, which were the engines of Germany’s war machine, could be more accurately attacked in the daytime.
Marshall of the Royal Air Force Sir Arthur ‘Bomber Harris’, however, had different ideas.
He conceived of a night raid so massive that it might even knock Germany out of the war (at least in conjunction with several other raids of a similar scale).
The British attacked by night to make it harder for the enemy to see them coming and shoot them down, but this raid would go a step further.
The 1,000-bomber force would be packed into a tight formation to further reduce casualties to the RAF, inflicting maximum damage in minimum time and thereby pulling off a huge propaganda coup.
Flak guns and fighters were the two countermeasures that would be deployed against them.
Both were terrifying obstacles for any bomber crew to encounter, but they were actually quite limited in scope.
Night fighter defensive boxes could only be deployed a maximum of six times per hour; likewise, flak guns could only be aimed at a certain number of targets. So the more targets that were presented all at once, the fewer gunners would be able to shoot down.
Thus, the regulation time-frame for a mission encompassing 1,000 bombers was reduced from four hours to 90 minutes.
It was a calculated risk to be sure. Collisions were more likely, but by extension, so was survival because the danger from night fighters and flak guns would be cut by almost two-thirds.
Despite this logic, Bomber Harris had trouble securing the planes he needed.
The Royal Navy were meant to augment the RAF with their Coastal Command aircraft, but they believed the propagandistic effect meant to be derived from the raid's massive scale was not reason enough to launch it.
So Harris scraped together planes and crews from Operational Training Units (OTUs), RAF Coastal Command and Flying Training Command and combined them with the 400 aircraft of Bomber Command for a combined force of 1,047 planes.
These were Avro Lancaster heavy bombers, of course, but also Halifax and Stirling heavy bombers and Wellington, Whitley and Hampden medium bombers.
On the night of May 30/31, 1942, this enormous air armada rumbled out of airfields in Britain and headed over the North Sea to Germany.
Cologne, Germany’s third largest city, was the target, chosen because it was comfortably within range and had good weather conditions.
In that 90-minute window over the city, the aircraft unleashed 1,455 tonnes of bombs, most of which were incendiaries.
The Germans fought back as best as they could, under the circumstances (a bombing mission of this size had never been launched before and must have taken them completely by surprise).
During the raid, they were able to down 43 British aircraft (they claimed it was 44), from flak and fighter aircraft (two were due to collisions).
The RAF, in turn, started 2,500 fires, 1,700 of which were classified as 'large' by German fire crews on the ground.
The city was severely hammered, with 3,300 non-residential buildings completely obliterated, 2,090 severely damaged and 7,420 lightly damaged.
‘Collateral damage’ was also severe, with 13,010 homes wiped out, 6,360 damaged severely, and 22,270 classed as lightly damaged. 411 people were killed.
Of course, the extent of the damage done by ‘Operation Millennium’, as it was called, was not definitively known at the time.
In 1945, with the war nearing its end, American troops entered the city and were greeted by the widespread devastation delivered three years before.
The History Channel reports that the once proud city had been reduced to “a heap of twisted iron and broken bricks. Blown up bridges, wrecked railways… gutted churches, shattered towers and shells of factories”.
Bomber Harris would come out of the war and go into the history books as one of the most controversial figures of the period. His devastating air raids have been classed as unnecessary by some historians.
No doubt scholars will continue to argue about the efficacy and morality of his strategy, but one thing is certain: By his own standards, Bomber Harris succeeded. His enormous attack may not have been the war-winner that he'd hoped, but it clearly did devastate the city of Cologne as he'd intended.
Cover image: The Bombing Raid on Cologne from National Archives UK.
For more on the Lancaster bomber read 'Bf 110 vs Lancaster 1942 - 45' by Robert Forczyk.