That's coolMartin certainly had plans for two stage R-2800s.
I'd have figured they could have designed it for twin-stage supercharging, but built the first variants with single stage-supercharging and just leave some empty space in the plane where the intercoolers would occupy.The Army chose the single stage R-2800 version with short wings, because that was the configuration that promised the best performance with the engines available at the time.
How much speed would the short-winged version do with a twin-supercharger? I ask because the XB-27 has bigger wings and that would throw off performance guesses.Other configurations had projected speeds in excess of 400 MPH, but required technology that was not yet available in 1939.