Quoting sources close to the French investigating authorities, the newspaper said that the co-pilot, 27-year-old Andreas Lubitz, appeared to have tried out a "controlled, minute-long descent for which there was no aeronautical reason" on the earlier outbound flight from Düsseldorf to Barcelona.
Investigators had discovered this after a close evaluation of the jet's black-box flight recorder, Bild said.
The French authorities are scheduled to publish an interim report on their investigation into the crash later on Wednesday.
French investigators believe that Lubitz, who had been diagnosed as suicidal in the past, deliberately brought the plane down.
Doctors had recently found no sign that he intended to hurt himself or others, but he was receiving treatment from neurologists and psychiatrists who had signed him off sick from work a number of times, including on the day of
Police found torn-up sick notes during a search of his apartment after the crash.