Responding to Great Britain's desperate need for fighter aircraft during WWII, GM's North American Aviation designed, built, and flew the P-51 Mustang in an incredible 150 days. In 1985, GM purchased Hughes Aircraft for $5.2 billion, only to unload most of it by 2003. However, it absorbed much of Hughes Electronics, whose expertise played a critical role in the development of the EV1.
Lotus developed an engine for small sporty aircraft in the early 1980s. It ran well but was never put into production, presumably because of company founder Colin Chapman's death.
Most WWII German fighter planes were powered by Mercedes-Benz 600-series V-12 engines. The firm also built engines for Zeppelins, including the ill-fated Hindenburg.
"A car from the wonderful folks who brought you the Zero" wouldn't have made for much of a U.S. advertising campaign. After the war, Mitsubishi built the MU-2, an exciting turboprop with a horrendous accident record.
Efforts to certify 356 and later 911 engines for aircraft use proved to be expensive failures.
Louis Renault was extremely interested in the pioneering French aviators and established an aviation department in 1907. Renault built V-12 aircraft engines until the 1950s for French military transports.
The "born from jets" slogan is simple bull -- Saab's first products were prop planes -- but there were vestiges of aeronautical practice in its cars, at least until GM made them into near-clones of Opels, Chevrolets, and Subarus. The now-separate Saab AB, meanwhile, continues to thrive as an aerospace company.
Toyota opened a light-aircraft department in California about twenty years ago and built an all-composite airplane using a Lexus V-8 engine.