In 1935–1936, he underwent primary pilot training at Randolph Field and was commissioned in the United States Army Air Corps.

In 1938, Baumler rejoined the U.S. Army as a second lieutenant, but in 1941 he resigned his commission again to join the American Volunteer Group ("Flying Tigers") then training in Burma.

He returned to the U.S. Army, and in December was assigned to the U.S. Military Mission in China, probably with the expectation that he would serve as AMISSCA's liaison to the AVG, which was in need of experienced staff officers.

He flew east on a Pan Am Clipper flying boat that was carrying tires and spare parts for the AVG fighter planes.

The cargo was dumped, and Baumler and the aircraft returned after it was strafed at Wake Island on the morning of December 7, 1941 – the Japanese attack on Pearl Harbor.

[citation needed] The end of the war resulted in the rapid demobilization of the Air Force and Baumler, lacking a college degree and with a history of drinking problems and former association with the Soviets, was turned down for a regular commission, and accepted a permanent rank of master sergeant in order to remain in the service.

It is said that during the Korean War he was responsible for a radar interception unit and from the way some Migs were manoeuvering he identified some of the Russian pilots as his former wingmen in Spain.