Boeing XB-38 Flying Fortress

Essentially an Allison-powered B-17E, the XB-38 was a project developed by engineers at Lockheed-Vega.  The ninth production B-17E 41-2401 had been delivered to Lockheed-Vega to help in setting up a Flying Fortress production line at their Burbank, California facility.  This was the aircraft modified as the prototype for the XB-38 design.
The standard 1,200 hp Wright Cyclone R-1820-65 nine cylinder radial engines were replaced with Allison V-1710-89 in-lines.  Both types of engines were turbocharged to improve performance at altitude.  The Allisons each developed 1,425 hp, an increase of 225 hp per engine over the Cyclones.
Boeing-Lockheed Vega XB-38. (U.S. Air Force photo)
Work on the project began in the Summer of 1942, but it was not until 19MAY43 that the XB-38 first took to the air.
The aircraft handled well during testing, but the project was delayed while problems with exhaust manifold leaks were corrected.
Boeing lacked an indoor paint shop at their Seattle facility which might explain the natural metal finish on 41-2401 when it was delivered to Lockheed-Vega. 
Four weeks into the testing program while on its ninth flight, the XB-38 developed a fire in the right inboard engine nacelle.  Efforts to extinguish the fire were unsuccessful and the aircrew bailed out.  The co-pilot was killed when his parachute failed to open.
While the Allison powered Fortress was slightly faster than the Cyclone powered version, it was also heavier and had a projected lower maximum ceiling.  In addition, the Allison engine was in demand for several USAAF Pursuit aircraft, including the P-38, P-39, and P-40 among others.  With no clear advantage to changing the design, the decision was made to continue Flying Fortress production with Cyclone engines.  Here is the XB-38 sharing the apron with a B-17F.
Markings were standard for the time, and included the “U.S. ARMY” lettering on the underside.  Noteworthy is the lack of defensive armament with only the dorsal turret having guns mounted.  The ventral ball turret is a dummy.  Interestingly, the sighting blister and scanning windows associated with the Sperry remote turret are still in place.
A beautiful profile view which shows off the contours of the Allison engine nacelles well.  Certainly an attractive aircraft!
From the front the aerodynamic streamlining of the engine nacelles is apparent.  Inline engines are generally heavier and require more maintenance than radials, but have a smaller frontal area which helps the designer reduce drag.

YB-40 gunship here: