A major German innovation was to use dive bombing aircraft, ‘sturzkamfflugzeu’, in close support of their front line troops. The Ju-87 or ‘Stuka’ became notorious as a terror weapon partly because it was fitted with a siren that wailed as the plane dived. It was a sturdy short range bomber capable of very accurate attacks on ground targets such as tanks or troop convoys and was also used against shipping. However it was vulnerable to other aircraft because it was so slow and could only be used confidently where the Germans had complete air superiority. In Poland it was able to operate almost completely unopposed.
Blitzkrieg: German divebombers support their army
Earlier in the war: S.S. Athenia – the first ship torpedoed in World War II
Later in the war: Athenia sinking "should have helpful effect" on US opinion