The boxer layout, with the crankshaft longitudinal and opposed cylinders, was developed by BMW Engineer Max Friz, and resulted in the 1923 BMW R32 — the first BMW motorcycle (yes, Virginia, BMW made motorcycles five years before it produced cars).

This engine layout always struck me as being utterly logical. The cylinders project sideways into the wind and have good primary balance, and transmission to a shaft final drive is relatively straight forward (or backward), eliminating any need for a bothersome chain or belt. Moreover, because the cylinders projected into the air stream, the engine on the opposed-twin Harley-Davidson XA ran 100° cooler than the V-twin Harleys.

During World War II, the U.S. Army asked Harley-Davidson to produce a motorcycle as good as BMW's side-valve R71. So Harley copied the BMW, simply converting metric measurements to inches, and produced the shaft-drive 750cc 1942 Harley-Davidson XA. Even though about 1,000 were made, the motorcycle never saw service.

The XA had many first for Harley-Davidson:

Opposed two-cylinder, air-cooled engine: Never repeated in a Harley-Davidson

Shaft drive: Never repeated in a Harley-Davidson

Welded frame: Introduced in larger production bikes about 1973.

First direct four-speed shift: Introduced in production bikes in 1955 K model.

Centrifugal Advance Distributor: Introduced in production bikes in 1964.

Spring and plunger rear suspension: Introduced in production bikes in 1956.

14 millimeter spark plugs: Introduced in production bikes in 1949.

Heavy duty Springer front fork: Similar to the 74c.i., but two inches longer for clearance.

Twin Linkert carburetors on a Boxer-style opposed engine other than racing cycles.

Left-hand throttle to allow for weapon discharge while riding.

Two 2.25 gallon gas tanks with separate shutoff valves.

Sealed beam headlight: Introduced on production bikes in 1949.

Positive ground electrical wiring

Click here to view a photo of the Harley-Davidson XA in Harley's own museum.

Here is the Harley-Davidson XA's story according to the American Motorcyclist Association Motorcycle Hall of Fame Museum:

“Blame the Jeep”

“Back in the early 1940s, the U.S. Army asked Harley-Davidson to design a next-generation military motorcycle. The company was already producing the WLA, based on its traditional 45-degree V-twin. But the Army wanted a bike with one feature the WLA didn’t have: shaft drive. For its target, Harley chose another well-developed military bike -- BMW’s R75, then in use by the German Army. Harley’s version, the XA, was a near duplicate, right down to the flat-twin engine.” (Note: Harley-Davidson copied the R71, not the R75.)

“The Army ordered a test batch of 1,000 XAs. At the same time, the military also asked Indian to make a 750cc shaft-drive twin, and it came up with a 90-degree V-twin design much like recent Moto Guzzis. The idea was to put both new machines through their paces, and award a lucrative military contract to the winner.

“The XA prototypes ... had a few significant developments beyond the copied engine. Breaking with H-D tradition, the throttle was on the left end of the bars and the hand clutch on the right, as specified by the Army. A massive rear rack would carry a then-lightweight 40-pound radio. And, starting in 1943, the XA also sported the company’s first telescopic fork. Mechanically, the large cooling fins stuck straight out in the breeze, reportedly keeping the XA’s oil temperature 100 degrees cooler than a standard Harley 45. At 4,600 rpm, the side-valve engine put out a claimed 23 horsepower.

“Eventually, the Army finished its testing, and decided that neither new bike would be built. Instead, they (sic) bought several thousand more Harley WLAs. Mostly, though, the U.S. military decided to hitch its hopes to a vehicle that could go through anything, didn’t tip over, and required very little training to operate -- the Jeep.

“The XA motor, despite its workable design, fizzled. Thus ended the Harley XA project -- and the idea of a tactical military motorcycle.”

The motorcycle shown below is the 750cc Indian 841, which was Indian's response to the Army's order. Its side-valve 90° V-twin opposed engine, which produced 25 HP (two more than the XA), suggests the later Moto Guzzi engines of that design.

The 841 also came with several other new features, including shaft drive, a foot shift, hand clutch, hydraulic girder spring fork, rubber-mounted handlebars, a sprung rear hub and 8-inch drum brakes, all of which made it onto postwar Chiefs.

Here is the Inidian 841's story according to the American Motorcyclist Association Motorcycle Hall of Fame Museum:

“1941 Indian Military Model 841
“The Wigwam's desert warfare bike”

“When the U.S. War Department asked American motorcycle manufacturers to come up with military machines for World War II, Harley and Indian answered the call.

“At first, both companies produced models with V-twin engines derived from their existing streetbike lines. But the War Department also asked for experimental designs to use for desert fighting in North Africa.

“Harley opted to copy the design of BMW’s R71—which was being used by the German Army in that hostile environment—building a flat-twin boxer motor.

“Indian took a slightly different route, creating the Model 841. The motor is a 750cc V-twin, which makes it sound like an American-style powerplant, except that the cylinders are turned sideways and set at 90 degrees. It’s an engine configuration that we now associate with Moto Guzzi—but Indian built it first. 

“The 841 also came with several other new features, including shaft drive, a foot shift, hand clutch, hydraulic girder spring fork, rubber-mounted handlebars, a sprung rear hub and 8-inch drum brakes, all of which made it onto postwar Chiefs.

“E. Paul duPont, who controlled Indian, had so much faith in the 841 design that he got one for his own use. He rode it around the country, stopping to show dealers and friends, and had grand plans for turning the military 841 into a civilian tourer.

“But none of that was to be.

“Although Indian built more than 1,000 examples of the bike and put it through extensive testing in the desert of California, the military never adopted the machine for wartime use. The same was true of Harley’s BMW-like XA. Instead, the primary motorcycle used by American forces was the 750cc V-twin Harley WLA, augmented by Indian’s 500cc V-twin 741 and the 750cc 640B, basically a military version of the Sport Scout.

“By the end of the war, even those motorcycles were being phased out as the army turned to Jeeps.

“This rare 841 (photo above), owned by Billy Doyle of Waterford, Michigan, was previously on display in the AMA's Motorcycle Hall of Fame Museum in Pickerington, Ohio.”

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