Bönsch studied mechanics at the State Trade School. He originally joined the army in 1915, but after completing basic training transferred to aviation. His original assignment was as a mechanic with Fliegerersatzkompagnie 6.
He requested flight training in 1917. He completed it at Flek 8 on 22 June 1917. His promotion to corporal followed six days later. In August, he was assigned to Flik 51 on the Italian front; he scored his first victory shortly thereafter, on 1 September,