Peanut butter gained popularity during the early 1900s, and at that time it was combined with a variety of ingredients, from peppers and watercress to various jellies. According to Bon Appetit, the first peanut butter and jelly recipe appeared in 1901. However, the two foodstuffs did not become the iconic duo as they're known today until the mid-1900s.
Peanut butter was popularized during the 1904 World's Fair, after which it was considered a sort of luxury item thanks to its novelty. Mass-produced, homogenized peanut butter hit the market in the 1920s, bringing humanity one step closer to the PB&J sandwich that has become a childhood and cheap-eating staple for millions of people, especially Americans.
World War II rations helped to popularize the combination of bread, peanut butter and jelly, as all three items were part of common rations for American soldiers during the war. These soldiers wanted to continue eating this wartime snack when they returned home, causing a sharp upswing in the consumption and sale of peanut butter and jelly as an inseparable duo..