DMG is the Daimler-Motoren-Gesellschaft, which was formed in Bad Cannstatt (near Stuttgart, Germany) by Gottlieb Daimler in November 1890. This company merged with Benz & Cie. in October 1926 to form the Daimler-Benz company.
Chief designer at DMG was Wilhelm Maybach, who was responsible for the first cars to be named Mercedes.
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Sources:
http://www.mbca.org/history.htm