On 10 September 1956, in the high desert above Edwards Air Force Base, North American Aviation's chief test pilot Bob Baker pushed the throttle of an aircraft nobody outside the U.S. Air Force fighter procurement office had ever heard of into afterburner. The airframe in front of him looked, from twenty feet away, like an F-100 Super Sabre. Closer inspection revealed something stranger. The air intake was not under the nose. It was on top of the fuselage, behind the cockpit, in a configuration so visually awkward that pilots immediately nicknamed the aircraft the “Ultra Sabre” or, less kindly, the “Man Eater” — because if the pilot ever needed to eject, the intake duct was right above his head.

Baker took the first YF-107A to Mach 1.03 in level flight that afternoon. It was the start of a six-month flight-test programme that produced one of the most overlooked Century Series fighters ever built — an aircraft that lost the U.S. Air Force's tactical-fighter-bomber competition to the F-105 Thunderchief in 1957, was retired immediately, and survives today in the form of two airframes in American museums. Almost nobody remembers it.

A Super Sabre on Steroids

The F-107 began life in 1953 as the F-100B — an evolution of the operational F-100 Super Sabre. North American's chief engineer for the project, Lee Atwood, started with the proposition that the U.S. Air Force's new tactical-strike requirement needed a single-seat fighter-bomber that could carry a nuclear store under the fuselage, dash supersonic to the target, and survive air-to-air combat over the target area. The F-100B was supposed to be that aircraft, scaled up from the F-100 Super Sabre with a larger engine, larger airframe, and a recessed fuselage hardpoint for the Mark 7 nuclear bomb.

By 1955 the design had evolved enough that the Air Force renumbered it the F-107A. The over-fuselage intake was the headline change. To make room for the recessed nuclear store, North American moved the air intake from under the nose (where it would have interfered with the bomb bay) to above the fuselage, behind the cockpit. The Variable-geometry Air Inlet Duct (VAID) was a sophisticated piece of mid-1950s engineering — it automatically opened and closed inlet ramps to optimise airflow to the J75 turbojet across the aircraft's entire speed envelope. The VAID worked. It was an early forerunner of the variable-geometry intakes later used on the F-104, F-15 and Concorde.

Performance That Mattered

The first prototype reached Mach 1.03 on its maiden flight. Within six months, the YF-107A was flying at Mach 2.0+ at altitude. The aircraft carried four 20mm M39 cannons, had wing hardpoints capable of carrying conventional or nuclear stores, and demonstrated an air-to-ground delivery accuracy in test that exceeded the Air Force's requirement.

It also handled well. Test pilot reports from Edwards in late 1956 describe the YF-107A as “responsive, predictable, and significantly easier to land than the F-100.” Cockpit visibility was excellent. The over-fuselage intake, despite its appearance, did not produce the engine ingestion problems the design's critics predicted. Bob Baker, three other North American test pilots and several Air Force test pilots flew it without major incident — though one of the three prototypes, 55-5119, was damaged in a forced landing after a hydraulic failure in 1957.

The F-105 Fly-Off

The Tactical Air Command ran a head-to-head competitive evaluation between the YF-107A and the Republic F-105 Thunderchief in early 1957. Both aircraft used the same Pratt & Whitney J75 engine. Both met the requirement. The F-105, however, had a recessed internal bomb bay rather than the YF-107A's recessed under-fuselage hardpoint, which meant the F-105 could carry larger conventional bombloads with less drag at high speed. The F-105 was also slightly more advanced in terms of avionics integration and easier to maintain.

In March 1957 the Air Force cancelled the F-107 programme. The three prototypes — 55-5118, 55-5119, and 55-5120 — were grounded as production aircraft. Two of them (55-5118 and 55-5120) were transferred to the National Advisory Committee for Aeronautics (NACA, soon to become NASA) for high-speed research. The third was used for ground testing and eventually retired.

What If

The F-105 went to Vietnam. By the end of the war, the Air Force had lost 382 F-105s in combat. The aircraft, designed for nuclear delivery at high speed and low altitude, was repurposed into a conventional fighter-bomber for which it was not optimised, and paid a brutal price. F-105 pilots flew the Wild Weasel SAM-suppression mission with weapons and tactics that were being invented week-by-week as the war unfolded.

The counterfactual question — what if the Air Force had chosen the YF-107A — never had a definitive answer. Some analysts argue the F-107 would have suffered the same losses, because the underlying problem in Vietnam was tactical (low-level penetration of dense air defence) not airframe. Others note that the YF-107A's recessed weapons mounting was more flexible for conventional ordnance than the F-105's bomb bay, and that the F-107 might have transitioned more naturally into the multi-role fighter the F-105 was forced to become. The honest answer is: nobody knows.

The Two Survivors

YF-107A 55-5118 lives today at the Pima Air & Space Museum in Tucson, Arizona, sitting outdoors in the high desert sun. YF-107A 55-5119 is at the National Museum of the United States Air Force at Wright-Patterson AFB in Dayton, Ohio, restored to flight-test livery. The third prototype, 55-5120, was scrapped at NASA Edwards after its high-speed research career ended in 1960.

Visitors to either museum sometimes stop in front of the F-107 and ask why the air intake is on top. The answer requires a small lecture on 1956 nuclear-delivery requirements, the geometry of the Mark 7 bomb, and the politics of Tactical Air Command procurement decisions. Most visitors lose interest after thirty seconds. The Ultra Sabre, sixty-eight years after Bob Baker took it to Mach 1 over Rogers Dry Lake, remains the Century Series fighter that almost nobody remembers. It deserved better.

Sources: Wikipedia — North American F-107; The Aviation Geek Club; Defense Media Network; SilverHawkAuthor; Skytamer; Jets 'n' Props; John Weeks F-107 Survivors website; Pima Air & Space Museum; National Museum of the USAF.