Constituted as 99th Bombardment Group (Heavy) on 28 Jan 1942. Activated on 1 Jun 1942. Trained with B-17's. Moved to North Africa, Feb-May 1943, and assigned to Twelfth AF. Entered combat in Mar 1943 and bombed such targets as airdromes, harbor facilities, shipping, railroads, viaducts, and bridges in Tunisia, Sardinia, Sicily, Pantelleria, and Italy until Dec 1943. Received a DUC for performance on 5 Jul 1943 when the group helped to neutralize fighter opposition prior to the invasion of Sicily by penetrating enemy defenses to bomb planes, hangars, fuel supplies, and ammunition dumps at the Gerbini airfield. Assigned to Fifteenth AF in Nov 1943 and moved to Italy in Dec. Flew long-range missions to attack such strategic objectives as oil refineries, marshalling yards, aircraft factories, and steel plants in Italy, France, Germany, Poland, Czechoslovakia, Austria, Hungary, Rumania, Bulgaria, Yugoslavia, and Greece.

Received another DUC for withstanding severe fighter assaults to bomb the vital aircraft factory and facilities at Wiener Neustadt on 23 Apr 1944. Other operations included assisting ground forces at Anzio and Cassino, Feb-Mar 1944; participating in the preinvasion bombing of southern France, Aug 1944; and supporting the Allied offensive in the Po Valley, Apr 1945. Inactivated in Italy on 8 Nov 1945.

Redesignated 99th Bombardment Group (Very Heavy). Allotted to the reserve. Activated on 29 May 1947. Inactivated on 27 Jun 1949.