Ralph Harmon to leave McDonnell Aircraft and take over management of the engineering efforts. He had previously worked for Beech Aircraft where he headed the design of the Beechcraft Model 35, one of the first all-metal general aviation monoplanes. He later worked on the larger Cessna 620 and McDonnell Model C119, but his interests lay with small aircraft, and he accepted Mooney's offer. He insisted on replacing the wood in the M20 with aluminum, and the all-metal M20B was completed by the end of 1960, less than a year after his arrival. In 1961 the company sold 222 M20Bs. The following year the M20C, a refined version of the M20B, was introduced; 336 were sold that year. Rachal, Hoffman, and Harmon were not experienced at running an aircraft factory, but saw the need to expand the product line and add dealers, and pushed ahead. In 1963 they introduced the M20D, essentially an M20C with fixed landing gear and a fixed-pitch propeller. This had a slightly lower price than the M20C and was intended as a basic or trainer model which would have lower insurance costs. It could be upgraded with retractable landing gear, and in fact most of them were upgraded over the years. The M20D was produced from 1963 to 1966 with a total production of 141 units. 1964 saw the introduction of the M20E Super 21, based on the M20C but with a more powerful fuel-injected Lycoming engine. The company sold 366 M20Es that year. To supplement the M20 line, Mooney began development of a high-performance pressurized single, the M22 Mustang. It first flew in September 1965 and was certified two years later. The project was ultimately unsuccessful. Sources disagree on the total production, some saying 36 units, others 32, 29 or 27. What is certain is that each plane was a financial loss for the company. It was reported as having a heavy control response and poor visibility, although it did fly high and fast. The name also could have caused problems, since Cavalier Aircraft had been modifying surplus North American P-51 Mustangs for civilian use and wrote a letter to Mooney asking them not to use the name "Mustang". The letter was ignored and Cavalier filed suit, but by that time the M22 line was nearing its end and the Mustang name had been dropped. During several years in the early 1960s, Mooney had discussions with