Folklore says: two professional recipe developers at Kellogg's created the Rice Krispie Treats to support a Campfire Girls troops fundraising effort.
Have you ever wondered what kickass human stumbled upon the glory that is the Rice Krispie Treat? Its butter. It's marshmallow. It's Rice Krispie. It comes together in 20 minutes or less. And, the homemade variety is 10x better than any store-bought kind.
In 1939, Mildred Day and Molita Jensen worked as modern-day recipe developers in the Kellogg's Test Kitchen. Throughout the 1900's most major food companies had a team of recipe developers staffed to create recipes utilizing their products. Some of the most famous and beloved recipes originate from these kitchen (Green Bean Casserole was invented by Campbells, Chocolate Chip Cookies by Toll House, etc.)
Back to the Rice Krispies at hand.
Kitchen folklore says it all started with Kellogg received a curious inquiry. A troop of Campfire Girls based in Kansas City were in desperate need of their next fundraising event. Ideally, they needed something easy, tasty and super delicious. The original recipe was less sweet than today's version and had added vanilla.
Mildred and Molita went to work. Eventually, they created the Rice Krispie treat. When the time came, Day went to Kansas City to support the fundraiser. She "brought along specially made baking trays and a giant mixer." The lore says she worked 16-hour days for 2 weeks straight to meet demand. The Rice Krispies Treats were a hit!
Kelloggs put the recipe on the cereal box in 1941 and the rest is history. It took another 50 years for Kelloggs to put out the prepackaged version.

Sources:
- Cookie trivia: Rice Krispie treats history. The Cookie Elf. (n.d.). http://www.cookie-elf.com/rice-krispie-treats.html#sthash.XfrIumAe.QxVZbkvL.dpbs
- Rice Krispies. In Wikipedia. https://en.wikipedia.org/wiki/Rice_Krispies
- (2017, September 21). From the archives: Love Rice Krispies Treats? You have an Iowan to thank. The Des Moines Register. https://www.desmoinesregister.com/story/entertainment/dining/2017/09/21/archives-love-rice-krispies-treats-you-have-iowan-thank/686025001/
- 1949 Rice Krispies ad