The Demon grew out of a 1949 US Navy requirement for a high-performance, swept-wing carrier fighter to succeed McDonnell’s own straight-winged F2H Banshee. The company delivered an aerodynamically clean, single-engine interceptor with a 35° swept wing fitted with leading-edge slats and trailing-edge flaps for carrier work. The XF3H-1 first flew on 7 August 1951 with McDonnell test pilot Robert Edholm at the controls.

The fatal decision was propulsion. The Navy directed that the Demon be built around the Westinghouse J40, a turbojet still in development and backed heavily by the service. When the J40 failed to deliver, the whole programme — a sound airframe — was dragged to the edge of cancellation before the switch to the Allison J71 rescued it. The Demon is a textbook case of a good aircraft nearly undone by an engine chosen for it.

Like several early-1950s Navy jets — including the F7U Cutlass — the Demon was designed around the Westinghouse J40, an engine the Navy expected to standardise across a generation of aircraft. It proved chronically underpowered and unreliable, one of the great engine fiascos of the era. On the J40, early F3H-1N Demons suffered repeated flame-outs and accidents; roughly six aircraft and four pilots were lost and the type was grounded, with the entire first production batch effectively written off. The programme survived only by re-engining with Pratt-era rival Allison’s J71 — a rescue that cost time, money and a black eye for Navy procurement.

The Demon’s greatest achievement was not what it did but what it led to. McDonnell used everything it learned building an all-weather, missile-armed carrier interceptor to design its successor. The F-4 Phantom II began life as an advanced development of the Demon — the aircraft the Demon “wanted to be” but could not, held back by a single engine and limited power. Where the Demon had one J71 and was firmly subsonic, the Phantom used two mighty General Electric J79s for reliability and Mach-2 speed. In a real sense the Phantom is a twin-engined, supercharged Demon — and it became the defining Western fighter of the Cold War.

The Demon was a mid-1950s US Navy programme retired in 1964, long before the era of published life-cycle cost data, and no consistent period flyaway unit price appears in the open record. Any precise dollar figure attached to the type today should be treated as an estimate rather than a sourced number. There is likewise no reliable cost-per-flight-hour figure for the F3H in open sources — unsurprising for an aircraft that left service more than sixty years ago.