For the Normandy campaign there were two air force units primarily concerned with supporting ground forces - the RAF's Second Tactical Air Force and USAAF's Ninth Air Force. These units operated light and medium bombers and fighter-bombers, and were trained to attack targets on and around the battlefield. Heavy four-engine bombers, such as the Lancaster and B-17, were operated by RAF Bomber Command and USAAF Eighth Air Force, and primarily flew strategic bombing missions on targets deep in enemy territory. Heavy bombers were, in general, not particularly accurate, but carried more and heavier bombs.

As the situation in Normandy bogged down, there were calls to use heavy bombers to support ground operations. These were resisted by the air forces, who believed that they were better employed against strategic targets, but a number of operations were flown by both Bomber Command and the Eighth Air Force. They had a mixed impact, the sheer weight of bombing having a considerable psychological impact on defenders, but inaccuracy led to friendly casualties from USAAF bombers during Operations Cobra and Totalise in July and August.

The attack on Falaise, Operation Tractable, was supported by Bomber Command (not just the RCAF, though the RCAF's 6 Group were part of the mission), operating unusually in daylight - the vast majority of their missions were flown at night. The crews had little experience in visually identifying targets, Bomber Command used Pathfinder aircraft to drop coloured target markers. For the Tractable mission, yellow target markers were to be used. Tragically nobody had communicated to Bomber Command that yellow smoke or flares were used by Allied ground units as friendly recognition signals - the tactical air forces were aware of them, but not the heavy bombers. Bombs dropped in the wrong area (or even the fear of such) caused ground units to deploy yellow smoke, in turn mistaken for target markers by subsequent aircraft. Blackburn talks about an Auster observation aircraft heroically trying to warn the bombers off by flying under them and firing red Verey lights, though this may unfortunately have had exactly the opposite effect as some crews reported bombing on red target indicators. In all 77 aircraft, 44 from 6 Group, out of 811 bombed in the area of friendly troops.

A report on the incident was written by Arthur Harris, AOC Bomber Command, available in Canadian Military History (Harris, Arthur T. (2006) "Report on the Bombing of Our Own Troops during Operation “Tractable”: 14 August 1944," Canadian Military History: Vol. 15 : Iss. 3 , Article 8.); see also Dr Seb Ritchie's Blue-on-Blue: Bomber Command and Operation Tractable, 14 August 1944. Ian Gooderson's thesis "Allied Close Air Support 1943-1945" includes a section on the use of heavy bombers along with a fuller account of the tactical air forces.