The division was formed by separating responsibility for Western Scotland (particularly the industrial areas of Clydeside and Ayrshire) and Northern Ireland from the existing 3rd AA Division, which continued to be responsible for the rest of mainland Scotland.

The HAA guns were concentrated in the Gun Defence Areas (GDAs) at Belfast and Glasgow, LAA units were distributed to defend Vulnerable Points (VPs) such as factories and airfields, while the S/L detachments were disposed in clusters of three, spaced 10,400 yards apart.

At the same time, experienced units were posted away to train for service overseas; in some cases they joined the 12th AA Division temporarily while they trained in Scotland; others remained with AA Command as unbrigaded units.

[4][39][40] At the end of 1941 S/Ls were declustered to form 'killer zones' for night fighters, and the S/L requirement for Northern Ireland was reduced to three batteries.

[42] During this period the division was composed as follows (temporary attachments omitted):[18][43][42][44] The increased sophistication of Operations Rooms and communications was reflected in the growth in support units, which attained the following organisation by May 1942:[42] The RAOC companies became part of the new Royal Electrical and Mechanical Engineers (REME) during 1942.