DCS World has a free P-51D Mustang trainer plane (I've forgotten what it's called, maybe the TF-51?) that you can fly in an open world mode, so maybe check that out but you'll need a pretty good PC. But it's a free game so there's no harm in trying.
IL-2 Sturmovik and Wings of Prey are other WWII sims you can try, Wings of Prey is the precursor to War Thunder and although it's not free, you will at least get a load of planes to fly from the start and there's a free flight mode too (which I don't think WT has). IL-2 is Russian-centric, too, so you get some planes you won't see often elsewhere.