Check this out - FSM has a review of an Academy P-51B. I don;t know if it's the same issue - but it says that it comes with an optional closed or open canopy. Plus it'll give you other info about the kit.
I built Academy's C version. Compared to the Hasegawa D and a Revell of Germany B that I've built, I thought it was the best of the bunch. Very nice detail all over the surface, in the wheel wells and in the cockpit.
I might have overdone it a bit on the weathering - it was an experiment.