02/28/2016. Remarks by Johan Visschedijk: "In August 1932 Northrop Aircraft Inc. completed the first Gamma all-metal single-engined low-wing monoplane and developed it in parallel with the similar Delta which first flew in May 1933. The Delta had a larger fuselage than the Gamma and was intended to meet airline requirements for a high-speed transport; but a restriction was put on the use of single-engined aircraft for passenger operations at night and over difficult terrain, and as a result most examples were privately owned or used for executive transport.

Only thirteen Deltas were built by Northrop - all Delta 1s, with the suffixes A, B, C, D, and E. They had faired-in non-retractable landing gears and most had six or eight passenger seats. They were fitted with a variety of nine-cylinder radial engines, including 775 hp Wright SR-1820 and 700 hp Pratt & Whitney Hornet radial engines. The pilot's cockpit was well forward and a fairing ran back from the cockpit to the base of the fin, and on later aircraft the depth of the fuselage was increased to incorporate a wider cockpit.

The first aircraft, the Delta 1A with a 710 hp SR-1820-F3 engine, was leased to Transcontinental & Western Air and used as a mailplane between Kansas City and Los Angeles, but it had an in-flight fire and crashed; the second, the Delta 1B with a 660 hp Hornet T2D-1 went to Aerovias Centrales in Mexico but that, too, was soon lost; the third, the Delta 1C, went to ABA (Swedish Air Lines), and was named Halland; the fourth aircraft, a Delta 1D, was an executive aircraft; and the fifth, the Delta 1E with a 660 hp Hornet, was much closer to the Gamma, having enclosed tandem cockpits over the trailing edge; it was used by ABA as a night mail aircraft.

The remaining eight aircraft were all Delta 1Ds and were mostly privately owned; one passed to the RAAF, some passed to Spain during the Civil War where one was used by the Spanish airline LAPE. The USCG purchased the twelfth-built Delta directly from Northrop, designated RT-1.

Canadian Vickers Ltd. acquired a manufacturing license and built twenty Deltas for the RCAF. Built in 1936, the first three became the Delta Mk.I and were used for photographic survey, later they were partially converted to Delta Mk.II standards and redesignated Delta Mk.IA. Fitted with provision for armament and downward-vision windows for the pilots, seventeen Delta Mk.IIs were built, of which the last eight, built in 1940, had larger vertical tail units (unofficially known as Delta Mk.III). The RCAF Deltas were operated with wheel, ski and float landing gears and the last six were withdrawn in 1945."