The 20th FG converted to North American P-51 Mustangs in July 1944 and continued to fly escort and fighter-bomber missions as the enemy retreated across France to the Siegfried Line. The group participated in the airborne attack on the Netherlands in September 1944, and escorted bombers to Germany and struck rail lines, trains, vehicles, barges, power stations, and other targets in and beyond the Siegfried Line during the period October–December 1944.