Preliminary development work for Jet propulsion was undertaken by Junkers in 1937, with design work for a full scale turbo jet engine beginning in 1939, followed by construction of the Jumo 004 A in early 1940. The first unit was run by December 1940, and several units were ready by the Summer of 1941. Initial flight tests, utilising a Bf 110, were carried out either at the end of 1941 or March 1942, and led to the 004 B, which incorporated several modifications.