The name given to a coordinated six-day long air offensive
launched against Germany
by the RAF Bomber Command
and the US Strategic Air Forces
After the introduction of the P-51 Mustang long range escort fighter, US Strategic Air Forces were prepared to end their ban on daylight operations, and do it in style. Starting on February 20th, over 3,800 USSTAF and 2,351 Bomber Command bombers unloaded over 20,000 tons of bombs on German fighter factories and related industries.
The Americans operated during the day, the British at night, and over the course of the operation the Americans lost 226 bombers and 28 fighters, while the British lost 157 aircraft. Despite the heavy losses, Big Week achieved its two primary goals. It set back German fighter production, and took out a number of irreplaceable Luftwaffe pilots.
Essentially, Big Week was intended to break the back of the Luftwaffe, and it worked. From then on, bombing raids encountered little airborne resistance, and the Allied landing at Normandy was almost completely unopposed from the air.