Le Mans, 1968, the way it was meant to sound: the rolling thunder of the 7.0-liter Ford GT40s, the screaming howl of the Ferrari P4s, the edgy rasp of the Porsche 908s, and the metallic snarl of the mid-engine, V-12-powered Jaguar XJ13. The thought of this impossibly low-slung British Racing Green roadster running wheel to wheel down the Mulsanne Straight with scarlet Ferraris, big-banger Fords, and scrappy Porsches is enough to get any enthusiast’s pulse racing. Of course, it never happened. But to wriggle into the one and only XJ13’s snug cockpit, grasp the well-worn wood-rimmed steering wheel, and fire up the fuel-injected V-12 nestling in the small of your back is to get a tantalizing taste of the glorious spectacle it would have been.

The XJ13 grew out of a program that started in the 1960s to develop a V-12 for Jaguar road cars. By 1964, a prototype quad-cam, 5.0-liter V-12 with SU carburetors had been fitted to a Mark X sedan for testing. From the beginning, engineers also worked on a race variant of the V-12 fitted with a newly developed Lucas mechanical fuel-injection system and a dry-sump oiling system. Chief engineer Bill Heynes, anxious to get the factory back into racing, sought approval from Jaguar boss Sir William Lyons to build a Le Mans prototype around the new V-12.

For the first time in Jaguar’s history, the powerplant would be located behind the driver. The mid-engine revolution started by Cooper’s World Championship-winning F1 cars in the late 1950s had spread to top-level sports car racing by the early 1960s, though Jaguar designer Malcolm Sayer reportedly had contemplated a mid-engine racing car as early as 1953. Back then, Jaguar technicians insisted on sticking with the tried and true front-engine layout for what became the D-Type, but the idea apparently knocked around Jaguar’s engineering office under the designation G-Type until 1958.

Despite talk of having the XJ13 ready in time for the 1965 Le Mans race, the project didn’t actually get underway until June 3, 1965—little more than two weeks before the race—and the car was not completed until March 1966. Then it was immediately grounded by Lyons, who decreed the XJ13 was not even to be driven around the factory grounds, much less tested and developed. Norman Dewis, Jaguar’s long-serving chief test driver, speculates the XJ13 ban was prompted by the decision of Lyons to merge Jaguar with the British Motor Corporation, which was publicly announced four months later.

Formal testing of the XJ13, therefore, did not begin until 1967. By mid-year, the 5.0-liter V-12 was developing 472 hp at 7,250 rpm, and a young David Hobbs had lapped the Motor Industry Research Association (MIRA) 2.82-mile banked track (an exaggerated tri-oval) just north of Coventry at 161.6 mph in the XJ13, a record not beaten until the Gordon Murray-designed McLaren F1 lapped it at 168 mph some 31 years later. Using a run-up, Dewis reached 200 mph on an old, two-mile-long World War II U.S. Army Air Forces airstrip at nearby Bruntingthorpe. The XJ13 was fast, though when Hobbs tested the car at Silverstone in August, he complained of poor brakes and a tendency to oversteer. But Jaguar’s Le Mans contender was never to turn a wheel on the racetrack in anger: The Le Mans race organizers announced motors for cars in the prototype class would be restricted to 3.0 liters for 1968. Instantly obsolete, the low-slung roadster was wheeled into a quiet corner of the Jaguar experimental engineering shop and left under a dust cover.

It sat there for more than two years until Jaguar’s public relations team had the bright idea of surprising journalists with the still-secret XJ13 at the launch of the V-12-powered Series III E-Type at the 1971 Geneva show. It mattered little that Jaguar’s first production V-12 differed in a number of significant ways from the experimental unit in the XJ13, most notably in terms of the cylinder heads, which had just a single, chain-driven overhead camshaft. Jaguar’s stillborn Le Mans racer was even yet a stunning-looking car capable of ferocious performance, and it would serve as a useful distraction from the slightly uncomfortable fact that the E-Type was by then basically a 10-year-old car.

To support the launch, it was decided to film the XJ13 at speed on the MIRA track in January 1971. The car was given a complete check and fitted with new wheels and tires that had been held in storage since the development program was canceled. Dewis did a number of laps at modest speeds for the cameras, then picked up the pace. On his third fast lap, at about 140 mph, the right-hand rear wheel collapsed. The XJ13 smacked the guardrail, then spun into the freshly plowed infield, bouncing into the air and rolling several times as the wheels dug into the soft, muddy soil. Miraculously Dewis, who was not wearing a seatbelt, escaped with a few bruises; the jockey-size driver had the presence of mind to switch off the ignition and dive under the scuttle to wedge himself in the cockpit when he realized he was merely a passenger. The XK13, however, was a mess, although a close examination revealed the basic monocoque was still sound, and the engine—a stressed member that carried the transmission and rear suspension—had remained in place undamaged, as the soft soil had absorbed the worst of the impacts.

In 1972, Jaguar managing director Lofty England authorized a rebuild when it was discovered the voluptuous forms—used by Abbey Panels, the Coventry-based shop that also built the first Ford GT40 chassis in 1963, to create the XJ13—were still in existence. The rebuild was completed in June 1973. Among the minor changes made to the car from the original spec was the fitment of a radiator and cooling fan from an XJ12 sedan and reshaping of the wheel arches. The XJ13 made its belated public debut on July 13, 1973, with a series of demonstration laps at Silverstone prior to the British Grand Prix.

It’s a steamy day at the delightful Monticello Motor Club track in leafy, upstate New York, as I fold my 6-foot-2-inch frame into the XJ13’s cramped cockpit. Norman Dewis was at least a foot shorter than me, and the XJ13 seems to have been tailor-made for him. I sit with my knees splayed on either side of the splintering wood-rimmed steering wheel, handmade by some forgotten craftsman in the Jaguar experimental workshop more than 40 years ago. I look around as technician Richard Mason gives me a preflight briefing: The XJ13 might be a one-of-a-kind car, a priceless piece of Jaguar history (Jaguar Heritage reportedly turned down an offer of $11 million for the car in 1996), but it’s far from a pristine trailer queen. The unfinished aluminum on the monocoque is scuffed to a satiny sheen, and there’s oil leaking into the cabin from somewhere. Faded Dymo labeling tape identifies the various switches arranged somewhat haphazardly across the dash to my left, and the acne under the paint at the base of the left-hand A-pillar betrays the repairs made in the aftermath of The Crash. The XJ13 might not have fulfilled its destiny, but it’s a car that’s lived.

I flick the ignition on and fire up the fuel pump, watching until the needle on the fuel-pressure gauge starts flickering wildly around 100 psi. A quarter travel on the gas pedal, then a flick of the ignition switch to its second position. The starter motor grinds…and suddenly the V-12 barks to life.

The XJ13 is, befitting its British heritage, right-hand drive, but the shifter is also on the right, mounted in the wide sill of the monocoque. The five-speed ZF transmission, with first gear on the dogleg back toward the driver, is a bit of a temperamental old bitch. I follow Mason’s instructions, selecting third, then second, before attempting to select first. The linkages sulkily comply, and the lever snicks home. We’re good to go.

Not wishing to make a fool of myself by stalling in front of Mason and the other Jaguar staffers on hand, I give the V-12 plenty of revs as I feed in the clutch to make my way out onto the track. There’s no need, really. This might be an old race car, but the V-12 is a sweetheart, surprisingly smooth and tractable and easy to handle at low speeds.

Back in the day, the XJ13’s test drivers would routinely take the big V-12 to 7,600 rpm and beyond, but we’re limited to just 5,000 rpm, not the least because this is the only complete gear-driven quad-cam V-12 engine Jaguar has left. (Mason says eight quad-cam engines were built during the early V-12 development program; only two had gear-driven overhead camshafts, and the rest had a cheaper chain drive system.) I take it gently at first, treating the XJ13 as if I’m waltzing a dear old aunt at a family wedding. But it doesn’t take long to realize that, more than 40 years ago, she was quite the party animal. And she doesn’t mind showing you she can still bust a move or two.

As the car warms up, it comes to life, shaking off its slightly arthritic demeanor. The steering is delightfully direct and lightens up nicely at speed. Like all V-12s, the engine loves to rev and doesn’t start to sparkle until the tach needle swings past the 3,000-rpm mark, the barking exhaust note developing a mean metallic snarl. Start to work it, and like all race cars, the XJ13 gets in the zone—everything starts to work in harmony. The brakes—a problem for Hobbs during testing—are fine for the speeds we’re doing, and the vintage-section Dunlop Racing tires, run at much lower pressures than back in the 1960s, deliver an impressively precise turn-in response and a surprisingly compliant ride. The XJ13 feels more like a modern sports sedan than a hardcore racer.

By the end of the day, I will have done more than 40 miles in the XJ13—for photos, video, and just the sheer joy of driving this wonderful old car. It’s sweaty, fatiguing work, though; the pipes connecting the front-mounted radiator with the engine run through the cockpit and give off heat like a furnace. Worse, the steering rack, a Series I E-Type unit, is mounted right behind the radiator, and the heat soaks right up the steering column and into the steering wheel. As our track session ends, the wheel is almost too hot to hold.

But I step out of this truly special Jaguar grinning from ear to ear. Though it never raced, it perfectly captures the last glorious gasp of an era when Le Mans racers could still be painted in their country’s national colors and aerodynamics was an elegant art rather than a cold science.

Could the XJ13 have won at Le Mans?

“I thought it had a lot of promise,” recalls Hobbs, a driver whose long, distinguished career includes stints in GT40s in the 1968 and 1969 24-Hour races. “The engine still stands out. [Jaguar special projects leader] Mike Kimberley had asked me to test the car because I had experience with Ford GT40s and Lola T70s, and it had the best engine of the lot. It was smooth as silk.”

Hobbs also thought the rest of the car still needed a lot of development, though. Four decades after he last drove the XJ13, he instantly lists the problem areas: “The ZF gearbox was not very good. It had old Dunlop disc brakes and Dunlop R5 tires on narrow rims—a set of modern tires would have made a staggering difference—and it had too much rubber bushing in the suspension. It moved around all over the place when you were braking late for a fast corner like the old Stowe Corner at Silverstone.” And Malcolm Sayer’s beautiful bodywork would probably have had to sprout all manner of wings and trim tabs by the time it made the grid at Le Mans. Hobbs says: “From what we learned with the Lola T70, the shape probably would have generated a lot of front-end lift.”

But Jaguars had won at Le Mans five times during the 1950s; the factory knew how to prepare fast cars that would go the distance. Had the Automobile Club de l’Ouest not changed the rules for the 1968 race, a churlish move that also kicked out the muscular 7.0-liter GT40s and the glorious Ferrari 330 P4s, a properly prepared XJ13 certainly could have been a contender.

Racing is full of coulda, shoulda, woulda stories. But Jaguar, with limited resources available, was probably right to focus on its road car development program instead. Little more than a year after it abandoned the XJ13 program, Jaguar would launch the XJ6, the car that would establish the template for the modern luxury sedan and help keep the marque alive through the dark, troubled days of the 1970s and early 1980s.

In many ways, the original XJ6 is a far more significant and influential car than the XJ13. But it’s impossible not to look at Sayer’s sensuously shaped sheet metal and that extravagant, snarling, quad-cam V-12 displayed like a gorgeous mechanical jewel under glass at Tiffany and wonder at what might have been.

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Engine 304.7-cu-in/4994cc DOHC V-12, Lucas mechanical fuel injection

Power and torque 502 hp @ 7600 rpm, 386 lb-ft @ 6300 rpm*

Drivetrain 5-speed manual, RWD

Brakes front: disc; rear: disc

Suspension front: control arms, coil springs; rear: control arms, coil springs

Dimensions: L: 189.4 in, W: 70.9 in, H: 39.3 in

Weight: 2,464 lb

*as of May 1967

“It’s not surprising I ended up working in the motor business,” Richard Mason, the technician entrusted with the care and maintenance of Jaguar Heritage’s precious XJ13, says. Mason was born and raised in Coventry, Britain’s Motown, and grew up in a house right near what was once the Humber factory. (Humber became part of the Rootes Group along with Hillman and Sunbeam. The Rootes Group was acquired by Chrysler in 1967, its remnants sold to PSA Peugeot-Citroën in 1978.) Ironically, the house in which Jaguar test driver Norman Dewis lived was a short walk away. Richard has been with Jaguar Heritage since the end of 2001.

Why I Like It: “It has its own mystique. It could have been one of the best all-time cars at Le Mans, and the passion of the people who created it—in secret—comes out when you drive it. It’s one of the top five cars in the world for its looks, and for what might have been.”

Why It’s Collectible: “That’s easy. It’s the only one of its kind. It’s almost like a national treasure.”

Maintaining: “The biggest issue is the fueling system. It’s a very early prototype Lucas mechanical fuel injection system. Regular oil changes are essential. The engine has a dry sump, and because it’s such a large-capacity system—7.5 gallons—you can’t be sure you get all the oil out, so you have to change it often to keep it fresh. And you have to check for corrosion because it’s all-aluminum. Otherwise, it’s much like maintaining any old race car.”