The 377MG Mini-Guppy was the first of the Guppys that was built using an entirely new fuselage to connect the existing Stratocruiser sections. This removed the Stratocruiser's limitation of a 2.4m wide floor, and allowed a 4m wide cargo floor and a maximum inside diameter of 5.5m with the constant section measuring just over 22m long. The aircraft featured a swing tail for loading cargo.

The first flight as 377MG Mini-Guppy, christened "Spirit of Santa Barbara", was on May 24, 1967. Two days later on May 26, the Mini-Guppy was used for its first cargo flight, flying a Budd Company Skylounge to the 1967 Paris Airshow. While in France, the Mini-Guppy was flown south to Toulouse for a demonstration for Aerospatiale officials.

377MG "Mini Guppy"
Engine: 4 x Pratt-Whitney R-4360, 2575kW        
Take-Off Weight: 64770 kg / 142794 lb
Empty Weight: 41275 kg / 90996 lb            
Wingspan: 43.0 m / 141 ft 1 in
Length: 38.7 m / 126 ft 12 in            
Height: 11.6 m / 38 ft 1 in
Cruise Speed: 400 km/h / 249 mph             
Crew: 3