On 4 September 1939, Bristol Blenheims of Nos. 107 and 110 Squadrons, RAF Bomber Command, swept low over the rain-shrouded North Sea to attack the German battle fleet off Wilhelmshaven. For Bomber Command, the raid marked the beginning of nearly five years of bloody conflict - five years that saw the Command's emergence from a nucleus of poorly-equipped, understrength squadrons into the most massive and formidable striking force in history. This is the story of three of those years, from the outbreak of hostilities in September 1939 to the end of 1942.