FG began operations with Lockheed P-38H Lightnings on 15 October 1943, and was the first to use these aircraft on long-range escort missions from the UK. The P-38H differed from earlier versions in being powered by 1425 hp Allison V-1710-89/91 engines. The Lightnings' engines were troubled by the addition of alcohol used as an anti-knock compound in their fuel supply; a British war economy solution which caused problems with water condensation on the ground and fuel line icing at altitude. Another British attempt to correct fuel composition caused lead metal deposits to coat cylinders and foul plugs throughout the squadron. The -H series Lightnings did not have adequate cooling for extended high-power usage, as their engine development had outstripped the cooling capacity of the integral intercooler which ran through the wing's leading edge. Pilots were instructed to restrict their periods of highest engine power to defined time limits, but many did not. As a result of these various influences, the Group's Lightnings suffered a high rate of attrition. Nevertheless, 55FG P-38H pilots provided cover for missions against aircraft plants during Big Week in February 1944. Lt. Col. Jack Jenkins led the group on 3 March 1944, when they became the first Allied fighters to reach Berlin on an escort mission. On 16 April 1944 the group moved to RAF Wormingford to accommodate the arrival of the 398th Bomb Group. The 55FG converted to North American P-51D Mustangs in July 1944, continuing their primary task of escorting B-17 and B-24 bombers that attacked such targets as industries and marshalling yards in Germany, and airfields and V-weapon sites in France. In July the group attacked gun emplacements during the Saint-Lô breakthrough in July 1944, and transportation facilities during the Battle of the Bulge, December 1944 – January 1945. The group also patrolled the air over the English Channel and bombed bridges in the Tours area during the invasion of the Continent in June 1944. The unit patrolled the Arnhem sector to support the airborne invasion of the Netherlands in September 1944 along with strafing trucks, locomotives, and oil depots near Wesel when the Allies crossed the Rhine in March 1945. The unit received a Distinguished Unit Citation for eight missions to Germany between 3 and 13 September 1944 when the group not only destroyed enemy fighters in the air to protect the bombers it was escorting, but also descended to low levels,