Forrest Mars had seen soldiers in the Spanish Civil War in the 1930s eating chocolate pieces with hardened, tempered coatings. The soldiers liked them because they didn't melt in their hands or pockets like plain chocolates. He developed his version of the candy when he returned from Europe and patented his process in 1941. He used the m & m initials for his own name and William F. R. Murrie, President of Hershey's Chocolate, who owned 20% of the product which were made with Hershey's chocolate. They became very popular with the soldiers of WWII and that popularity has grown ever since.