In the North Sea (which besides the Atlantic and the Mediterranean was one of the operational theatres in which the Sundelands were most used) the large British flying boats often clashed with an enemy aircraft which was about the same buisiness,the German Dornier Do 18D,which was smoller,less powerful and less well-armed but nonetheless useful. The origines of this twin-engine flying boat went back to 1934 : Deutsche Lufthansa had asked Dornier to develop a more modern successor to the Wal, to be used for transatlantic mail service. Six Dornier 18s were entrusted to the German airline between 1935 and 1937 and because of its exceptional range the aircraft aroused the interest of the Luftwaffe as well, who ordered a military version. This was designated the Do 18D-1 and went into service in 1938, staying in the front line until 1942. Under illustration by Shigeo Koike is DORNIER Do 18E "ZEPHIR" Mail serviceflying boat.