Sills graduated from the University of Kansas in 1955, and worked for the U.S. Navy as an aviator from 1955 to 1958. Following his work flight testing at Boeing and McDonnell-Douglas, he held numerous engineering positions with the Cessna Company in Wichita. Sills made the initial flight of the Cessna Citation business jet in 1969. He received the University of Kansas distinguished Engineering Service Award in 1992 for leadership in design of the Citation fleet of business jet aircraft. He served on the University of Kansas aerospace engineering advisory board, and the Society of Automotive Engineers Aerospace Council. As an airline transport pilot rated in Learjet, Citation 500, 650, and 750, Sills received numerous awards for his outstanding work in the engineering of business jet aircraft.