Its large diameter wheels, fitted with brakes, were on a centrally hinged split axle held between main legs with oleo shock absorbers and near-horizontal drag struts. For some time this wheeled gear was replaced by floats. The first flight was made on 2 April 1927 and shortly afterwards the prototype was named the "Maiden Milwaukee". At the end of the month it was flown to Washington D.C. where it was on display at the Pan American Aero Congress. In 1927 it took part in the National Reliability Tour. Flown by Randolph Page, it won the second prize of $2,000. In September, flown by John H. Miller it gained second prize at the National Air Races held at Spokane. It also gained prizes for efficiency (first place) and speed (third) in the Detroit News Air Transport Race and first prize for efficiency in an Aviation Town & Country Club of Detroit Trophy contest. These later events brought $1,850 in winnings. Despite the safety and efficiency awards the H-18 did not sell and only the prototype was built. By 1929 it had been converted by Jesse C. Johnson into an unusual rotary wing type with a driven rotor under each wing. After a string of all-metal prototypes, McDonnell's 1928 H-45 and H-47 used a thinner, externally braced wing, more powerful engines, had two more seats in a side-windowed cabin and were much more successful.