From late 1941, the Stirling played a pioneering role in the formation of the RAF's Pathfinder squadrons, specialist navigation and target-finding squadrons to assist Main Force squadrons. From the spring of , the number of Stirlings in service began to increase. From May 1943, raids on Germany were often conducted using over a hundred Stirling bombers at a time. Stirlings were amongst the RAF bombers used during the First 1,000 bomber raid against Cologne. Norris observed that, by 1942, the type had "given plenty of punishment to the Germans and was also proving that it could itself take punishment to an incredible extent". There were several incidents in which damaged aircraft, such as one Stirling which suffered a head-on collision with a Messerschmitt Bf 109 fighter over Hamburg, were able to return to base.