It's quite possible they were American planes - especially if he saw them during daylight. In the weeks leading up to D-Day the RAF and the USAAF hammered communication and transport links so as to make it difficult for the Germans to get men and supplies to Normandy (and, as a ruse, Calaise and Norway). When the USAAF struck deep into France or Germany they would have fighter escort that would fly part of the way and then rotate back as the next group came to take their places. The pilots then had licence to hit targets of opportunity on the way back, which often meant flying on the deck looking out for trains, motor cars, airfields, columns of troops and so on. These fighter support aircraft were both USAAF and RAF - usually Mustangs, Spitfires and Thunderbolts (although after the invasion the latter were usually kept for ground attack work in support of the allied armies). I seem to remember the Germans also had a fleet of Condor anti-shipping aircraft at Merignac and no doubt assorted fighters. It would make sense to attack these aircraft on the ground so they could not be moved north to attack the invasion fleet. This may explain the presence of American aircraft in the Dordogne valley.