[1] Sloniger joined the United States Army Air Service on November 7, 1917[2] and was sent to the University of California, Berkeley for officers training.

After earning his wings, he was shipped out to the Issoudun Aerodrome in France where he flew Nieuport[3] and SPAD[4] pursuit planes.

[5] He then spent time flying the Tampico Oil Company payroll, in the form of cash and gold bars, from the US to Mexico.

In 1927, Sloniger was hired at Robertson Aircraft Corporation flying the CAM 2 mail route from St. Louis - Peoria - Springfield and back.

[4][7] The Air Mail scandal of 1934 mandated that the airlines reorganize into new, more efficient organizations, which led to Sloniger being named American's first chief pilot.

Before the war, American had been a domestic airline only, and Sloniger volunteered for the chance to fly international flights to Europe.

[6] Matson Navigation Company decided to start up a luxury airline serving Hawaii in an attempt to break up Pan Am's international route monopoly.

Gann wrote about their maiden flight to Honolulu in his memoir Fate is the Hunter with, "My respect for Sloniger was enormous and so I had no objection whatsoever to flying as his co-pilot.

[8] He was a member of the American Airlines retired pilots group the Grey Eagles and the Quiet Birdmen.