From 1927 to 1929 he participated very prominently in several US national and international balloon races, most notably winning the prestigious Gordon Bennett Cup with co-pilot William Olmstead Eareckson in June 1928.

In August 1929 he was commissioned as test pilot of the radical metal-hulled airship ZMC-2, newly completed at Grosse Ile, Michigan.

After a successful series of evaluation flights, he flew the airship in September of that year to what was to become its sole home base at Lakehurst, New Jersey, arriving without mishap except for a small perforation in the envelope which press reports of the time claimed to be the result of a pot-shot en route from someone on the ground.

[5] However, the attempt nearly ended in tragedy when the balloon envelope ruptured near maximum height, sending the spherical pressurized gondola plunging earthwards.

Prior to leaving for Europe, Kepner had been a key figure in demanding increased fuel capacity in aircraft like the P-38 Lightning and P-51 Mustang.

When he insisted they comply, complaints by the companies to Material Division (Wright Field) resulted in a threat to have charges laid against him for tampering with aircraft specifications.