When the United States Army Air Forces was established, there were 4 types of Bombardment Squadrons:

Bombardment Squadron (Dive)

Bombardment Squadron (Heavy)

Bombardment Squadron (Light)

Bombardment Squadron (Medium)

In 1943, a Bombardment Squadrons (Dive) equipped with two engine aircraft became Bombardment Squadrons (Light), but those flying single engine planes became Fighter-Bomber Squadrons. Starting the same year, the type appearing in parentheses was replaced by a type following a comma. For example Bombardment Squadrons (Heavy) became Bombardment Squadrons, Heavy.

In 1944 the type Bombardment Squadron, Very Heavy was added. During the war there were additional type designations that applied only to a few specialized units.

Bombardment squadrons were numbered 1-100, 301-655 and 668-885. The numbers 101-300 were reserved for National Guard units. The numbers 656-667 were dive bomber units constituted in the spring of 1943, but that action was revoked and the units were not activated. The 391st through 436th Bombardment Squadrons were former Reconnaissance Squadrons. Due to reorganizations, disbandments, and the like, some numbers were used more than once and some squadrons had more than one number as a bombardment squadron.