Upon arrival in the European Theater of Operations the group became part of IX Fighter Command.
 The group's station in England was RAF Winkton, an unimproved field, where it received 75 Republic P-47 Thunderbolts by 16 April.
 By 1 May, it was ready to fly its first combat mission, a fighter sweep over Normandy.
Later in the month, it began to fly Noball missions, strikes on V-1 flying bomb launching sites.
For the rest of the month, the 404th helped prepare for Operation Overlord, the Normandy invasion by bombing and strafing targets in northern France.
 The group's squadrons provided top cover for landings in Normandy on 6 and 7 June 1944.

On 6 July the 404th moved across the Channel to an Advanced Landing Ground at Chippelle Airfield, France.
The group also flew interdiction and escort missions, strafing and bombing such targets as troop concentrations, railroads, highways, bridges, fuel and ammunition dumps, armored vehicles, docks, and tunnels, and covering the operations of Boeing B-17 Flying Fortresses, Consolidated B-24 Liberators, and Martin B-26 Marauders that bombed factories, airdromes, marshaling yards, and other targets.
 On 6 and 7 June, the group provided top cover for the Normandy landings.
 The 404th provided close air support to ground troops, supporting the Allied breakthrough at Saint-Lô from 29 through 31 July 1944, provided continuous cover for four allied armored divisions despite severe losses.
 For this action the group received the French Croix de Guerre with Palm from the French government.

On 10 September 1944 the group flew three armed reconnaissance missions, attacking factories, rolling stock and communications centers despite adverse weather and heavy flak, for which it was awarded the Distinguished Unit Citation.
On 28 September 1944, 2/Lt John W. Wainwright was credited with destroying six enemy aircraft on a single mission, three of which were knocked down in a midair collision during a dogfight.

The 404th provided close air support to ground troops, supporting the drive through Belgium and the Netherlands in September 1944.
For its actions supporting the liberation of Belgium, the group was thrice cited in the Order of the Day by the Belgian government, earning it the Belgian Fourragere.
From 21 to 25 January 1945, it attacked German armor and transportation withdrawing from the Battle of the Bulge, and claimed to have destroyed or damaged over 1000 enemy vehicles.

During February 1945, it assisted with the reduction of German forces west of the Rhine and the widening of the Remagen bridgehead east of the Rhine during March 1945.

When the war in Europe ended, the group assisted with the disarming of the Luftwaffe and dismantling of the German aircraft industry
Following its return to the United States, the unit was reassigned to the Third Air Force in the United States and prepared for deployment to Okinawa to take part in planned Invasion of Japan.
When the Atomic bombings of Hiroshima and Nagasaki took place and the Pacific War suddenly ended, these plans were canceled.
Most personnel either separated from the military or were reassigned to other units, while a skeleton staff arrived at Drew Field, Florida on 1 September.
The unit was inactivated on 9 November 1945.
Upon arrival in the European Theater of Operations the group became part of IX Fighter Command.  The group's station in England was RAF Winkton, an unimproved field, where it received 75 Republic P-47 Thunderbolts by 16 April.  By 1 May, it was ready to fly its first combat mission, a fighter sweep over Normandy. Later in the month, it began to fly Noball missions, strikes on V-1 flying bomb launching sites. For the rest of the month, the 404th helped prepare for Operation Overlord, the Normandy invasion by bombing and strafing targets in northern France.  The group's squadrons provided top cover for landings in Normandy on 6 and 7 June 1944.
On 6 July the 404th moved across the Channel to an Advanced Landing Ground at Chippelle Airfield, France. The group also flew interdiction and escort missions, strafing and bombing such targets as troop concentrations, railroads, highways, bridges, fuel and ammunition dumps, armored vehicles, docks, and tunnels, and covering the operations of Boeing B-17 Flying Fortresses, Consolidated B-24 Liberators, and Martin B-26 Marauders that bombed factories, airdromes, marshaling yards, and other targets.  On 6 and 7 June, the group provided top cover for the Normandy landings.  The 404th provided close air support to ground troops, supporting the Allied breakthrough at Saint-Lô from 29 through 31 July 1944, provided continuous cover for four allied armored divisions despite severe losses.  For this action the group received the French Croix de Guerre with Palm from the French government.
On 10 September 1944 the group flew three armed reconnaissance missions, attacking factories, rolling stock and communications centers despite adverse weather and heavy flak, for which it was awarded the Distinguished Unit Citation. On 28 September 1944, 2/Lt John W. Wainwright was credited with destroying six enemy aircraft on a single mission, three of which were knocked down in a midair collision during a dogfight.
The 404th provided close air support to ground troops, supporting the drive through Belgium and the Netherlands in September 1944. For its actions supporting the liberation of Belgium, the group was thrice cited in the Order of the Day by the Belgian government, earning it the Belgian Fourragere. From 21 to 25 January 1945, it attacked German armor and transportation withdrawing from the Battle of the Bulge, and claimed to have destroyed or damaged over 1000 enemy vehicles.
During February 1945, it assisted with the reduction of German forces west of the Rhine and the widening of the Remagen bridgehead east of the Rhine during March 1945.
When the war in Europe ended, the group assisted with the disarming of the Luftwaffe and dismantling of the German aircraft industry
Following its return to the United States, the unit was reassigned to the Third Air Force in the United States and prepared for deployment to Okinawa to take part in planned Invasion of Japan. When the Atomic bombings of Hiroshima and Nagasaki took place and the Pacific War suddenly ended, these plans were canceled. Most personnel either separated from the military or were reassigned to other units, while a skeleton staff arrived at Drew Field, Florida on 1 September. The unit was inactivated on 9 November 1945.
137th Special Operations Group