Richard wrote
Whirlwind was used in prototyping SAGE after it was
built. However, I
don't consider that to "predate SAGE" as its just part of the SAGE
design effort.
That's like claiming that the MX-774 ICBM was just part of the Apollo
spacecraft design effort. Whirlwind was designed and partially
constructed before the SAGE project was initiated. Whirlwind was not
originally conceived as a SAGE prototype; it was used as one because it
was available and has some (but not all) of the desired characteristics
of a computer for such a system.
Eric