Donley has been the service’s top civilian for the last five years and was the acting secretary for seven months prior, making him the longest-serving secretary in Air Force history.

During his tenure, he oversaw a large-scale reduction in Air Force personnel driven in part by budget cuts. “His leadership came during a challenging time for the Air Force, and he helped instill a culture of responsibility, initiative, and professionalism to the service,” Defense Secretary Chuck Hagel said in a statement.

Donley did not immediately announce his future plans, but Hagel said he planned to return to “private life.”