Curtiss P-40 was a workhorse fighter for the US in WWII. It was in service at the start of the war and soldiered on through the finish. But advances in aeronautics rendered it obsolete well before the end of hostilities.
That did not stop Curtiss from upgrading the design. And though it didn't reach squadron service, the P-40Q did end up as the prettiest of the P-40 line. Though they missed out on the fighting, the airplanes ended their days as racers.
Who can resist a big in-line engine, sleek lines and such a rich family tree?
The LOOTM for February 2011, the Curtiss P-40Q.