In 1931, a 26-year-old at Procter & Gamble wrote a 3-page memo that created the product manager role. Almost nobody knows his name. But every PM job description you've ever read still runs on his idea: one person who owns a product end to end, close to the customer. That was Neil McElroy. He handled the advertising for Camay soap. He put a new idea in a memo on May 13, 1931: a "brand man." One person responsible for a single brand, start to finish. His memo told that person to "study the…