[5] Starting in 1938, Mattern was a Lockheed test pilot on the P-38 Lightning and during the war helped develop the 'Piggyback' two-seat version that significantly reduced training accidents.

In 1946 he was diagnosed at the Mayo Clinic with a ruptured blood vessel in his brain (perhaps due to repeated excessive G-forces experienced while demonstrating P-38s) and was unable to fly again because of the condition.

After losing his ability to fly, Mattern and his wife became real estate brokers and then operated a travel agency.

On May 18, 1937, the day after his divorce was finalized, Mattern was married to Dorothy J. Harvey, a showgirl, at Berwyn Methodist Church.

This failed round-the-world flight attempt ended in an emergency crash landing at Borisov, Belarus, USSR, on July 7, 1932.