We were doing incremental development as early as 1957, in Los Angeles, under the direction of Bernie Dimsdale [at IBM's ServiceBureau Corporation]. … - Gerald Weinberg

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We were doing incremental development as early as 1957, in Los Angeles, under the direction of Bernie Dimsdale [at IBM's ServiceBureau Corporation]. He was a colleague of John von Neumann, so perhaps he learned it there, or assumed it as totally natural. I do remember Herb Jacobs (primarily, though we all participated) developing a large simulation for Motorola, where the technique used was, as far as I can tell, indistinguishable from XP.