The group conducted 340 bombing missions with the Eighth Air Force over Europe, operating out of RAF Bassingbourn.

The 91st received orders to deploy overseas and on 24 August 1942, the ground echelon entrained for Fort Dix, New Jersey, where it remained until 5 September, embarking on the RMS Queen Mary.

Arriving at Greenock, Scotland, on 11 September, the ground echelon moved by train to RAF Kimbolton, a war expansion airfield in the English Midlands.

The group lost one of its 35 bombers during transit when a 401st B-17 crashed in fog into a hillside near Cushendall, Northern Ireland, killing 8 of the crew and a flight surgeon.

Intended as a light or medium bomber field, its runways were not suitable for the combat weights of B-17s fully loaded with bombs and fuel.

Three practice missions in as many days indicated to the staff of the 91st that the runway would quickly deteriorate and Colonel Wray immediately consulted Col. Newton Longfellow, VIII BC commander, who suggested Wray inspect the RAF Bomber Command OTU base at RAF Bassingbourn, Cambridgeshire (52°06′N 00°03′W / 52.100°N 0.050°W / 52.100; -0.050), to see if it might be suitable.

Not only was the base more appealing from its closer proximity to London, but it had been constructed in 1938 and was considerably more comfortable, with permanent brick buildings, including barracks for enlisted personnel (in contrast to the Nissen huts at Kimbolton), landscaped grounds with curbed roadways (Kimbolton, like many war-time fields, was noted for muddy conditions); and had already been re-constructed to a Class A airfield.

The 91st began combat operations on 4 November 1942, when it received a field order for a mission to bomb the submarine pens at Brest, France, later changed to an attack on the Luftwaffe airfield at Abbeville.

These circumstances were typical of those encountered daily by all the heavy bomber groups in the autumn of 1942 as they pioneered the concept of strategic bombing by daylight.

Four (91st, 97th, 301st and 303d) had been earmarked for the Twelfth Air Force in support of Operation Torch and were in England to acquire combat experience and stage for forward movement to North Africa.

As late as 15 December the impending transfer of the 91st BG to Algeria was postponed because of logistics difficulties and a shortage of airdromes in North Africa.

On 17 April the group led the Eighth Air Force on its first mission against the German aircraft industry, attacking Bremen.

The Eighth developed in the next three months into a force of sixteen B-17 groups and began attacking industrial targets deep inside Germany beginning at the end of July.

On 17 August 1943, the 91st Group led a mission to bomb the ball-bearing factories at Schweinfurt, Germany, losing 10 aircraft.

The 91st won its second DUC as part of the six-group task force attacking the AGO Flugzeugwerke assembly factory (license-building the Focke-Wulf Fw 190A) at Oschersleben, Germany, on 11 January 1944.

The first attack by the 91st on Berlin came on 6 March, when it led the entire Eighth Air Force at a loss of 69 bombers (6 of them from the 91st), followed by half a dozen more to the German capital in the next two months.

Aided by the use of radar-equipped Pathfinder force bombers, the 91st BG averaged a mission every other day for the remainder of the war.

Beginning 16 March 1944, the 91st began receiving replacement B-17's that were by a change in USAAF policy no longer painted olive drab, and the bomber force became almost completely "natural metal finish" by July 1944.

The 1st Combat Bomb Wing, of which the 91st was a part, adopted the use of a red empennage and wingtips in June 1944 to more easily identify its groups during assembly for missions.

The intensity of operations during this phase is reflected by the 100 B-17's lost by the 91st Group during 1944, compared to 84 in 1943, despite the diminution of the Luftwaffe during the spring and summer.

Radar-directed flak became very proficient in defending critical targets and the fighter force hoarded its pilots and fuel for occasional mass interceptions of the bombers.

Suffering several losses to intense flak, for which this target was notorious, the 91st found itself isolated from the bomber stream at the division rally point, where it was attacked by large numbers of Fw 190A-R8 sturm fighters of IV./JG 3.

[5] Distinguished Unit Citation World War II: The air echelon left Bassingbourn on 27 May 1945, and moved to Drew AAB, Tampa, Florida.

The organization was redesignated as the 91st Operations Group on 29 August 1991, and activated at Minot Air Force Base, North Dakota.

The squadrons of the 91st OG are: Two 91st B-17's survive, one at the National Museum of the United States Air Force at Dayton, Ohio and the other at the Steven F. Udvar-Hazy Center in Chantilly, Virginia.