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Service Groups or Air Service Groups as they were called were a "new model" or Thomas Plan group.

The B-29 units in the Pacific were the first to get these units. As you mentioned, some maintenance men (armorers, crew chiefs, etc) during WW II were assigned to the combat squadrons, and there was a group engineering officer. These folks performed what was called 1st echelon maintenance. More complicated tasks were called 2nd, 3rd, and 4th echelon maintenance. The primary function of the new model Service Groups (soon renamed Air Service Groups) was to perform second and some third echelon maintenance for one combat group.

Typically the BG anfd ASG would be assigned to the same wing, When a maintenance task was too complex for the bomb group to perform, it went to the ASG. Things that were even more complicated would go to an Air Depot unit.

Air Service Groups
90th Air Service Group Main Page
90th ASG Roster
507th Air Material Squadron Roster