The McDonnell Douglas D-3135: the first commercial blended wing body from Long Beach

It's been known to me ever since I was a teenager that beginning in the late 1980s McDonnell Douglas investigated designs for a blended wing body (BWB) airliner, viewing a BWB concept as boasting more fuel efficiency than a tube-and-wing airliner. Notwithstanding the fact that Boeing continued work on a large BWB aircraft after acquiring McDonnell Douglas and built the subscale X-48 vehicle to test the flight characteristics of a BWB with not just commercial but also military applications, the BWB designs from McDonnell Douglas were not the first commercial designs for blended wing body aircraft that the company designed. In the 1970s, McDonnell Douglas first toyed with the idea of a blended wing body when it designed a so-called spanloader aircraft as a potential rival to the Boeing 747's hold on the commercial air freight market.

Artwork of the McDonnell Douglas D-3135 spanloader freighter with containers being loaded into the wing (the conventional D-3133 "Nation Builder" design is shown in the background). 

In October 1975, under contract from NASA, McDonnell Douglas began undertaking concept studies for a spanloader airplane to be used for long-haul commercial freight, and the company designation D-3135 was applied to the McDonnell Douglas spanloader commercial freighter design studies. The D-3135 was 202 feet 6 in (61.72 meters) in length with a wingspan of 285 feet 5 in (87 meters), a height of 73 feet 8 in (22.45 meters), a wing area of 18,314 square feet (1,701.4 m²), and a gross takeoff weight of 1,350,000 lb (612,350 kg). It resembled the Boeing Model 759-100 and 759-121 spanloader proposals in marrying a straight wing having 20% percent thickness with a conventional tailed layout and a slim fuselage, and it featured wingtips canted outwards at 18 degrees. Power came from six individually podded 58,000 lb (258 kN) thrust Pratt & Whitney JT9D turbofans situated above and ahead of the wing's leading edge, and the D-3135 was to carry 637,000 lb (288,940 kg) of freight housed in 42 intermodal containers, which were loaded into the wing via the wingtips. To compartmentalize its gross weight with tarmac infrastructure, the D-3135 had 16 main landing wheels below the center fuselage and two sets of four outrigger landing wheels below the outer wing sections.

Alternate McDonnell Douglas flying wing spanloader freighter design (courtesy of NASA)

In tandem with the D-3135, McDonnell Douglas also worked out a flying wing spanloader freighter similar in appearance to the flying wing iterations of the Boeing 759. It was 225 feet (68.6 meters) long with a wingspan of 311 feet 8.16 in (95 meters), a height of 67 feet 3.08 in (20.5 meters), a wing area of 14,896 square feet (1,384 m²), and a gross takeoff weight of 1,115,746 lb (506,102 kg). The thick wings protruded from an abbreviated center section including the cockpit, and a pair of vertical stabilizers with top-mounted vertical stabilizers were situated along the wingtips. The flying wing spanloader design would carry 600,000 lb (272,155 kg) of cargo housed in 32 intermodal containers, and power was provided by six 40,000 lb (177.9 kN) thrust turbofans situated above and ahead of the leading edges of the wings. As with the D-3135, the flying wing spanloader would have four sets of four main landing wheels below the center section and two sets of outrigger landing gear with four wheels each.

Despite offering greater freight capacity than the 747 cargo versions, none of the McDonnell Douglas spanloader designs progressed beyond the design phase, and even if the D-3135 had been built, its outrigger landing wheels would not been compatible with narrow-gauge runways, in which case the design probably might have needed to take off from airfields without long, narrow runways. Today, the JetZero company headquartered at the city in California where the D-3135 was designed is developing the Z5 blended wing body airliner project and testing a prototype for the Z5 concept, and if flight tests of the subscale and full-scale technology demonstrators for the JetZero design are successful, it is not implausible that the Z5 could potentially be adapted into a purpose-built commercial freighter about the same wingspan as the D-3135.

References:

Cox, G., and Kaston, C., 2020. American Secret Projects 3: U.S. Airlifters Since 1962. Manchester, UK: Crécy Publishing.

Gunston, B., 1991. Giants of the Sky: The Largest Aeroplanes of All Time. Sparkford: Patrick Stephens Limited.