The USAF service cap for Field-Grade officers features several design elements that set it apart from the caps worn by Enlisted Airmen and Company-Grade officers. Most prominent is the silver bullion used for the embroidered Air Force “Darts and Clouds” symbol; the bullion is embroidered onto black broadcloth that covers the visor. A closer observation also reveals that the Great Seal of the United States, worn as a cap device, is rendered without a circle.
While the Air Force uniform manual refers to the Men’s Service Cap as a “Wheel Cap,” it offers no explanation for why the term is employed—which understandably has led to a lot of speculation among Veteran and active-duty Airmen alike. Not surprisingly, many suspect the alternative appellation has something to do with the cap’s nickname as the “bus driver’s hat” since bus drivers man the wheel in the course of their duties.
But it’s highly unlikely that the Air Force would casually insert a phrase into an official manual that could be construed as derogatory or an insult. Another, simpler explanation is probably closer to the truth: this style of cap was first worn as a Fatigue cap in the Army to replace the tall Shako caps imported from European armies in the early 19th century, and to the soldiers the round crown simply looked like a wheel.