While Boeing was working on the XB-15 in the 1930s that would ultimately lead to the smaller but more advanced B-17 Flying Fortress and B-29 Superfortress, the company was engaged in a secret parallel project to develop a flying wing bomber that used some of the XB-15 features but was expected to exceed the performance of the XB-15 in every aspect. Boeing has never been previously associated with flying wing research with its current efforts in BWB (blended-wing-body) designs and the X-48 research program the legacy of the merger with McDonnell Douglas. Conventional wisdom of the day at Boeing regarded flying wings as less stable and having less than ideal handling issues than aircraft of conventional layout. Despite the prevailing attitudes of the company at the time, it did engage in flying wing research in 1935, developing a whole series of flying wing aircraft from the XB-15-derived flying bomber to fighters, airliners and even flying boats.
The Boeing Model 306 was the biggest and most impressive of the designs, aiming to capitalize on the aerodynamic advantages of the flying wing to improve upon the design work started with the XB-15. Having a crew of 10, the Model 306 had a swept wing of 140 feet in wingspan with a fuselage 60 feet in length. Four forward-mounted engines on the wings used 850-horsepower Allison V-1710 12-cylinder liquid cooled engines. Having a fully-retractable taildragger landing gear layout, the Model 306 bomber would have had a range of 5,000 miles with a 2,500 bomb load. Defensive armament consisted of a mix of 50-caliber and 30-caliber guns.
One of the more interesting features of the Model 306 had its elevons supported behind the wing trailing edge on long struts. Wind tunnel testing showed this arrangement more efficient as the pivoting and use of the elevons wouldn't interfere with the aerodynamic performance of the swept-wing.
It was only in 2006 that this series of designs came to light. It appears that all of the flying wing designs were collectively designated in-house as Model 306. Considering the state of the art in aircraft design in the 1930s, the Boeing Model 306 flying wing bomber stands out as quite futuristic!
Source: Secret Projects: Flying Wings and Tailless Aircraft by Bill Rose. Midland/Ian Allan Publishing, 2010, p71-71.