The Si 204 was planned as a small all-metal passenger aircraft with two crew and eight passengers for German airline Deutsche Luft Hansa (DLH). Development was initiated in 1938. The contractor was, as usual, the RLM, but the development was conducted in close collaboration between DLH and Siebel in Halle.
It originated in the Klemm-Flugzeugwerke Halle that had been founded in 1934 as a branch of Leichtflugzeugbau Klemm in Böblingen. Its name changed to Siebel Flugzeugwerke when it was taken over by Friedrich Siebel in December 1937. After World War II the company was revived as Siebel Flugzeugwerke ATG (SIAT) in West Germany in 1948, with its headquarters in Munich. In 1956, its headquarters were moved to Donauwörth and the company became WMD-Siebelwerke ATG (WMD/SIAT) in 1958 in cooperation with Waggon- und Maschinenbau GmbH Donauwörth (WMD). In 1968 the company was absorbed by Messerschmitt-Bölkow-Blohm after MBB became the major shareholder