William Diehl was born on June 9, 1933, in Tulsa, Oklahoma. He was commissioned a 2d Lt in the U.S. Air Force through the Air Force ROTC program at Oklahoma A&M College (now Oklahoma State University) on January 25, 1956, and went on active duty beginning March 12, 1956. Lt Diehl next completed pilot training and was awarded his pilot wings in May 1957, followed by service as an instructor pilot before receiving an Air Force Institute of Technology assignment to complete his Master's Degree at Oklahoma State University from 1960 to 1962. He served as an Engineering Specialist with Headquarters Air Force Missile Test Center at Patrick AFB, Florida, from August 1962 to February 1963, and then attended Missile Range Technician training at Patrick AFB from February 1963 to January 1964. His next assignment was as a Chemical Engineer with Headquarters Air Force Systems Command at Andrews AFB, Maryland, from March 1964 to January 1966. Capt Diehl served as a flight examiner with the 1001st Operations Squadron at Andrews AFB from January 1966 to March 1967, followed by F-105 Thunderchief Combat Crew Training from March to August 1967. His next assignment was as an F-105 pilot with the 469th Tactical Fighter Squadron at Korat Royal Thai AFB, Thailand, from October 1967 until he was forced to eject over North Vietnam and was taken as a Prisoner of War on November 7, 1967. He was reported to have died in captivity the next day, November 8, 1967, but his remains were not returned to the United States until March 6, 1974. Capt Diehl was officially listed as Missing in Action until February 1, 1973, during which time he had been promoted from Captain to Lieutenant Colonel.

His 2nd Distinguished Flying Cross w/Valor Citation reads:

Major William C. Diehl Jr distinguished himself by heroism while participating in aerial flight as an F-105 Thunderchief Pilot over North Vietnam on 27 October 1967. On that date, Major Diehl was a member of a flight assigned to attack a high priority military target. Through under almost constant missile attack, and flying through intense anti-aircraft fire he delivered his bombs precisely on his target, severely damaging the immediate area. The outstanding heroism and selfless devotion to duty displayed by Major Diehl reflect great credit upon himself and the United States Air Force.