Who was the first to define the RAM model?
According to the chapter notes of Chapter 1 of the book "Computational Complexity: A Conceptual Perspective" by Oded Goldreich, 


  The RAM model is attributed to von Neumann's report [234].


where [234] refers to 


  J. von Neumann. First Draft of a Report on the EDVAC, 1945. Contract No. W-670-ORD-492, Moore School of Electrical Engineering, Univ. of Pennsylvania. Reprinted (in part) in Origins of Digital Computers: Selected Papers, Springer-Verlag, pages 383--392, 1982.