This change was primarily made due to the rapid advances in aircraft (and aircraft bombing technology) that took place between 1920 and 1939. During World War II, the Bombardier Badge was a common aeronautical badge of the United States Army Air Forces. The badge was issued to all military bombardiers upon completion of basic flight training and advanced bomber instruction. With the creation of the United States Air Force in 1947, the Bombardier Badge was gradually phased out and none were issued after 1949.
The modern-day Navigator Badge is considered the U. Air Force successor to the Bombardier Badge. Please let me know if there's anything else I can do for you!