What engines could the B-17 use?

With almost plodding regularity, B-17 Flying Fortresses went to war powered by Wright R-1820 radial engines. It was a good fit and served the B-17 well.

But the Fortress airframe dallied with a variety of other powerplants in sometimes radical modifications.

The original Model 299 prototype established itself as a contender in 1935 with Pratt and Whitney R-1690 Hornet nine-cylinder radial engines developing 750 horsepower.

All follow-on B-17s, starting with 13 service test Y1B-17s, were propelled by Wright R-1820 Cyclone variants that grew from 920 to 1,200 horsepower before combat models made that the norm. 

In 1942, Lockheed had access to an early B-17E during the process of establishing a Lockheed Vega assembly line for building B-17s under contract. Negotiations with the Army Air Forces that year resulted in a contract to modify this B-17E by replacing its radial Wrights with four liquid-cooled Allison V-1710-89 engines. The V-1710 was principally a fighter engine, powering the P-38, P-39, P-40, and other pursuits.

The swap promised an increase of about 200 horsepower per engine. The modified B-17E airframe received an entirely new alpha numeric designation as the XB-38. It carried a dummy Sperry ball turret in place of the smaller remote turret it had as an early B-17E. This was done to make airframe drag more comparable with that of a current production B-17 for evaluation purposes.

The liquid-cooled Fortress first flew on May 19, 1943. The XB-38 promised a cruise speed more than 25 miles an hour faster than a standard B-17F, but the Allisons gave the XB-38 a lower service ceiling, at only 29,600 feet.

Testing with a Lockheed crew was far from finished when, on June 16, 1943, the Number Three Allison engine caught fire and would not be extinguished. As flames threatened to reach wing gas tanks, the two-man crew bailed out at 25,000 feet over California’s San Joaquin Valley.

The unmanned XB-38 came to earth near Tipton, California.

Pilot George MacDonald died when his parachute failed to open, while pilot Bud Martin survived, with injuries.

The XB-38 project died with the crash of the aircraft. It is emblematic of the many hurry-up ideas created, especially early in the war, when not enough time had elapsed to give total confidence in the unfolding production and combat scenario. Ultimately, the B-17 Flying Fortress would earn its reputation powered by Wright radial engines to the very end of combat.

At war’s end, the need for test bed aircraft that could carry new turbojet and turboprop engines aloft spawned a pair of B-17G conversions by Boeing, given the model designation 299Z. These B-17s kept all four R-1820 radial engines, and incorporated a series of streamlined engine mounts in the nose to accommodate turboprops, advanced versions of the R-3350 piston engine, and even an underslung turbojet.

In November 1947, Pratt and Whitney bought two B-17Gs from Air Force surplus at Altus, Oklahoma. One became a spare parts source and the other, serial 44-85734, was ferried to Boeing in Seattle for modification. The work included moving the cockpit aft about four feet to accommodate large test engines in the nose.

At Boeing’s Wichita, Kansas, plant, an Air Force-owned B-17G that was on bailment contract to Curtiss-Wright received similar modifications.

The changes on these test beds included adding structural members and thicker skin on some parts of the fuselage to take the loads expected to be imposed by the fifth engine in the nose.

The beauty of the big B-17 engine test beds was their ability to take a test engine aloft, with the safety margin of all four original engines intact. Publicists were fond of photographing the test beds in flight with all four R-1820s shut down, the only power keeping the plane aloft coming from the tested turboprop in the nose.

Curtiss-Wright made one unusual, yet predictable, change to its test bed B-17, replacing its regular Hamilton-Standard propellers with Curtiss Electric props and hubs. The Curtiss-Wright aircraft tested the Wright Typhoon XT35 turbine engine originally envisioned for a Boeing bomber that became the B-52 with different engines. This test bed also flew with an underslung Curtiss-Wright J65 turbojet.

A nose-mounted T49 propjet variant of the J65 was also flown. The T49 turboprop was applied to the experimental Boeing XB-47D version of the Stratojet bomber.

Curtiss-Wright bought the B-17 from the Air Force and flew tests, including a turbo-compound version of the R-3350 piston engine. 

Pratt and Whitney used its test bed B-17 for the T34 turboprop engine. Originally of interest to the Navy, the T34 ultimately found a home powering the Air Force Douglas C-133 transport instead. Placed in storage by the early 1960s, the Pratt and Whitney test bed was resurrected to test propellers for the Navy in 1965 and 1966.

In 1967, the Pratt and Whitney 299Z was passed to the Connecticut Aeronautical Historical Association near Hartford. On outdoor static display, the 299Z featured an empty nose mount that still conveyed its special purpose as an engine test bed.

Disaster struck on Oct. 3, 1979, when a freak tornado touched down among the museum’s parked aircraft and flung a Grumman Albatross onto the B-17. The museum divested itself of the wrecked Fortress, and portions of it, along with parts of another B-17, went into the creation of the B-17G called “Liberty Belle,” which unfortunately suffered a forced landing and fire in 2011. 

An entrepreneurial air tanker company, Aero Flite, took a fresh look at re-engining the B-17 in 1970, stripping four Rolls Royce Dart turboprop engines from a Viscount airliner and applying them to B-17F N1340N, Tanker 34. The conversion placed the long Dart turboprops far forward of the propeller line of a standard B-17.

Aero Flite had only a short time to fly its turbine B-17. On Aug. 18, 1970, the modified bomber crashed while fighting a fire near Dubois, Wyoming. The NTSB called the mishap a stall/mush accident, with the bomber hitting trees during the post-drop pull-up on a downslope, downwind run. The crew was killed.

The Curtiss-Wright 299Z was retired by the early 1960s, ultimately becoming part of Arnold Kolb’s Black Hills Aviation air tanker service in Spearfish, South Dakota. Kolb also bought a studio B-17G fuselage that had appeared in television episodes of “Twelve O’ Clock High,” and he ambitiously removed the foreshortened nose of the 299Z and spliced the stock B-17G nose back in place.

Kolb lacked hangar space at Spearfish to accomplish this precision job, so it was undertaken outdoors on the ramp. He said the warming sun would heat and expand one side of the B-17 more than the shaded side, causing a plumb bomb used for keeping the job aligned to move laterally. This perplexed the crew until the cause was determined. 

Kolb’s reconstituted B-17 air tanker, N6694C, served for years until a 1980 mishap in North Carolina burned the nose section. Parts of this B-17 may be used in future restorations.

There’s another, lesser known five-engine B-17 testbed, made from B-17G 44-85747. It retained its stock cockpit position and carried an XT40 turboprop and later the T56 turboprop, both from Allison.

The melodious Wright Cyclone motors of a stock B-17 confirm their ability to power this classic bomber. But the airframe proved capable of adapting other engines for other purposes.