By 1944 the Republic P-47 Thunderbolt was already one of the biggest, heaviest single-engine fighters of the war — a seven-ton brute that pilots adored. Then Republic asked a wild question: what if we made it even bigger, and bolted on the most powerful piston engine America ever built? The answer was the XP-72, the Super Thunderbolt, and for a brief moment it was one of the fastest propeller fighters on Earth.

It never fired a shot in anger. The jet age caught it first.

A fighter built around an engine

The heart of the XP-72 was the Pratt & Whitney R-4360 Wasp Major: a colossal 28-cylinder radial arranged in four rows, producing around 3,450 horsepower.

To feed that engine enough air at altitude, Republic mounted the supercharger behind the pilot, driven by a long extension shaft — the same trick used on the P-47. The rest of the aircraft was essentially a Thunderbolt grown to superhuman proportions. The first prototype flew on 2 February 1944 with a conventional four-bladed propeller; the second, completed that June, swung a pair of contra-rotating propellers to absorb all that power without simply torquing the aircraft off the runway.

Blistering performance

The numbers were spectacular for a piston fighter. In testing the XP-72 hit roughly 504 mph, with a ferocious rate of climb that made it a natural high-altitude interceptor — exactly the kind of aircraft you would want to catch fast bombers or, as the British were then discovering, incoming V-1 flying bombs.

Impressed, the Army ordered 100 production P-72s. On paper, the Super Thunderbolt looked like the ultimate expression of the piston-engine fighter.

Overtaken by the jet

Timing killed it. By the time the XP-72 was flying, the war had shifted: what the Army Air Forces needed most was long-range escort fighters, not short-legged interceptors. And on the horizon was something the XP-72 could never beat — the turbojet. Republic’s own jet-powered P-84 Thunderjet was already coming, and it promised far more growth than any piston engine ever could.

The production order was cancelled. Only two XP-72 prototypes were ever built, and neither survives. The Super Thunderbolt became a footnote: the magnificent end of a line, arriving just as that entire line went obsolete.

It is one of aviation’s recurring stories — a machine perfected exactly when perfection stopped mattering. The XP-72 was arguably the finest piston fighter Republic ever designed, and precisely for that reason, the world had already moved on.

Sources: Vintage Aviation News; HistoryOfWar.org; Old Machine Press; Wikipedia (Republic XP-72; Pratt & Whitney R-4360).