Blended wing body design

Blended-wing-body (BWB) designs, such as for the B-2 warplane, feature no clear distinction between the aircraft's fuselage and wings, and optimised aerodynamics. A passenger aircraft with this design may be 20% more efficient than a conventional tubular design.
BWB design is a proven technology, though it still faces many barriers to adaptation for passenger aircraft. Introducing a clean sheet design aircraft would disrupt common industry practice of "incremental improvements" to existing models, and incur much higher development costs and uncertainty. The high wingspan-to-height ratio of BWB design makes it only suitable for large aircraft, meaning that development costs cannot be split over a model family of different sizes. Other challenges are passenger acceptance for an aircraft without windows and incorporating emergency exits in a theatre-like seating layout.
An alternative design is the double-bubble - developed by MIT, however this was bought by Boeing's drone program in 2017 with few additional statements or details since.
In Europe, Airbus presented a demonstration prototype (a small model) at the Singapore air show in 2020. It has been under testing since 2019, demonstrating that this concept continues to receive interest. Airbus also estimates an energy efficiency improvement potential of 20% relative to current aircraft design.
Boeing, together with NASA, tested BWB passenger aircraft models on small-scale prototypes (Boeing X-48).