The airspeed indicators in US planes were in mph. We have several original P-51 instument panels as wella s panels from many WWII aircraft, and all the ASIs were in mph. Our museum has two flyable AT-6s and there must be another 4 - 6 around the airport at Chino, and all have or had ASIs in mph. Some of the P-51s have been retrofitted with ASIs in knots because ATC assigns you speeds today in knots, but the originals were in mph. I am not sure about the museum P-51s because they are flyable and so we don't do much "restoration" work on them.
To tell the truth, I have not looked at a Corsair, Bearcat, Avenger, or other Navy plane with enough attention to notice what units their ASIs are calibrated in, but I will this weekend since I am now curious.
We have an Avenger, an SBD, a Corsair, a Skyraider, a Ryan FR-1, as well as some Navy jets. I assume the later planes were in knots, but probably won't look at any planes that aren't flying since their cockpits may or may not even be complete enough to HAVE an instrument panel. Static planes sometimes are VERY incomplete inside. We have a Gloster Meteor, for instance, but it is a shell. It is complete from the outside but there are no engines, no hydraulic lines, and nothing inside the cockpit. You'd never KNOW that unless you got a ladder and looked inside.