'Millennium' was a British air raid by bombers of Air Marshal Sir Arthur Harris’s RAF Bomber Command against Köln (30/31 May 1942).

The undertaking was the first 'thousand-bomber raid', and caused considerable devastation in the German city, which was eventually bombed in 262 separate raids by the Allies, including more than 31 major raids by Bomber Command.

'Millennium' was the most notable of these raids, and as the first 'thousand-bomber raid' was launched for three reasons. Firstly, it was expected that the destruction wrought by such raids might be enough to knock Germany out of the war. Secondly, the early 'thousand-bomber' raids were useful propaganda for Allied nations and neutral nations (and also the Germans). Thirdly, it was expected that such a large number of bombers flying in a steady stream through the radar-directed defences of the Kammhuber Line would serve to overwhelm the German night fighter arm and so reduce the number of bombers destroyed.

At this stage of the war Bomber Command had a regular first-line strength of only some 400 aircraft. But by pressing into service both men and machines from training units as well as 250 aircraft of Air Chief Marshal Sir Philip Joubert de la Ferté's RAF Coastal Command and four aircraft of Air Marshal Sir William Welsh’s RAF Flying Training Command, Harris knew he could concentrate more than 1,000 aircraft for this single effort. Just before the raid took place, however, the Royal Navy refused its permission for RAF Coastal Command’s aircraft to be involved in the raid on the grounds that the real threat of the U-boats which were Coastal Command’s defined targets in the Battle of the Atlantic was more somewhat important than any propaganda coup.

By crewing 49 aircraft with pupil pilots, however, Harris was able to commit 1,047 bombers to the raid, this figure being some 2.5 times that of any of the command’s previous raids. In addition to the bombers attacking Köln, another 113 aircraft were used for intruder raids against the airfields used by German night-fighters.

'Millennium' was the first occasion in which the tactic of a bomber stream was used, and most of the tactics used in this raid would remain the basis for standard Bomber Command operations for the next two years, with some of the elements retained for the rest of the war. The recent introduction of 'Gee' radio navigation equipment allowed the bombers to fly a given route at a given time and height.

The British night bombing campaign had been in operation for some months, and a statistical estimate could be made of the number of bombers likely to be lost to German night-fighters and Flak, and how many would be lost through collisions. Minimising the former demanded a densely packed stream, as the controllers of a night fighter in a three-dimensional box could direct a maximum of only six interceptions per hour, and the Flak gunners could not concentrate on all the available targets at once.

Earlier in the war four hours had been considered acceptable for a mission, but in this raid the bombers would all pass over Köln within 90 minutes rather than the figure of four hours which had earlier been considered acceptable. It was anticipated that the concentration of bombing over such a short period would overwhelm the city’s fire brigades and so cause great conflagrations similar to those inflicted on London by the Germans during the 'Blitz' of 1940/41.

In the event 868 aircraft bombed the main target, and another 15 attacked other targets. The bombers dropped 1,455 tons of bombs, two-thirds of which were incendiaries. This started 2,500 separate fires, of which 1,700 were classed by the city’s fire brigades as large conflagrations. The action of fire fighters and the width of the streets stopped the fires combining into a self-sustaining firestorm, but most of the damage was nonetheless caused by fire rather than explosion. Some 3,330 buildings were destroyed, 2,090 were seriously damaged and 7,420 were lightly damaged for a total of 12,840, of which 2,560 were industrial or commercial structures. The sole military installation damaged was the barracks of the Flak gunners. The number of persons reported killed was between 469 and 486, of which 411 were civilians and 58 military. Another 5,027 persons were listed as injured and 45,132 as bombed out. It was estimated that from 135,000 to 150,000 of Köln’s population of nearly 700,000 fled the city after the raid.

The RAF lost 43 aircraft, which equated to 3.9% of the 1,103 bombers sent on the raid, of which 22 aircraft were lost over or near Köln, 16 of them brought down by Flak and four shot down by night fighters, two lost in a collision, and two Bristol Blenheim light bombers on attacks on night fighter airfields.