In May 1940 he was aide de camp to the Dutch supreme commander, general Henri Winkelman. He refused to give his word of honour not to harm German interest and became a POW. He was conversant in cosmography and advanced mathematics, and he lectured interested Dutch and British prisoners at Colditz Castle on both, in particular he taught geodesy to Patrick Reid. While in Colditz, he invented a device which, when attached to a micrometer, could obtain measurements accurate to within a tenth of a millimetre of any lock. He was therefore able to manufacture a key to fit any such lock in Colditz. He lectured other prisoners on how to use this device correctly, a course that lasted six months. On 9 September 1942 van Doorninck and British Lieutenant H. N. Fowler became one of the lucky few who escaped Colditz. Slipping with four others through a guard office and a storeroom dressed as German officers and Polish orderlies, they managed to make it out of the Castle. The others were unfortunately recaptured and only van Doorninck and Fowler reached Switzerland.
- Foreword by D.J. van Doorninck to the Dutch translation of the book by Reinhold Eggers, Colditz, the German story, 1961, 1974 Dutch translation.
- Leo de Hartog: officieren achter prikkeldraad 1940-1945; Hollandia 1983
- Van Doorninck genealogy at Geneanet
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