In its general design, the Northrop Delta low-wing transport monoplane, developed for long-range, high-speed cargo and passenger service, is similar to the Northrop Alpha transports now flying the Los Angeles-New York overnight air mail (in 1933). It had, however, more than 45 per cent greater payload capacity and increased performance for a given horsepower. These features were the result of extended structural developments and wind tunnel investigations. Refinements of structural details were made which, combined with the use of the new 24 SRT Alclad instead of 17ST formerly used, provided an all-metal full cantilever airplane of 45 per cent greater gross loading than the Alpha with only a small increase in the empty weight.

In the Delta model, the pilot was placed forward of the passenger compartment and just to the rear of the firewall to provide maximum visibility" The cockpit is covered with a sliding enclosure. The fuselage diameter is large enough to permit a seating arrangement of either two or three abreast, an aisle between.

Features of easy servicing and maintenance include a quickly detachable rubber-bushed alloy steel engine mount, jigged inter-changeable cowling. Fully enclosed ball bearings on ail controls and oversize bushings and joints at all points of wear.

Stability is positive in all directions enabling a level course to be maintained for long periods without touching the controls. Full lateral control is maintained beyond the stalling point of the wing, even with flaps down.

A reduction in landing speed and length of glide has been obtained by the use of split trailing edge flaps. With the flaps in full down position tire maximum lift of the airfoil is increased 35 per cent and the drag by 300 per cent. The combination extends the usefulness and safety of the ship as many small fields, hitherto considered unsafe, can be flown into and out of by the average pilot.

The fuselage is of all-metal stressed skin monocoque structure with longitudinal stiffeners and large oversize reinforcing rings. Permanent jigged assembly of the fuselage and, center section forms a rigid integral unit of these two parts. Six gasoline tanks of 340 gallons capacity are installed in the center section, permitting long range operation when desired without encroaching upon cargo space within the fuselage.

The attractively finished cabin provides accommodations for eight passengers and contains approximately 200 cubic feet of available space. The cabin is 57 inches wide and 60 inches high and the fully upholstered seats are 19 inches wide, with an aisle between offering comfort and ample leg room. Individual windows, reading lights, ash trays and ventilation control are provided at each seat. A heating and ventilation system, controlled from the cockpit, admits into the cabin either heated air or air at outside temperature. The cabin is unusually quiet, due to the use of sound-deadening insulation throughout.

The passenger compartment entry door is on the left side of the fuselage and an emergency exit is available from the forward right side of the ship. A lavatory may be installed by eliminating one of the rear seats.

Thirty-five cubic feet of space is available for cargo in a compartment aft of the cabin. A Wright Cyclone SR 1820 F-3 rated 710 hp at 5,500 feet altitude or a P. & W. Hornet of similar horsepower rating is used. These engines use 87 octane gasoline, so when a lower octane fuel is desired there will be a small decrease in performance corresponding to the decreased horsepower output. The metal propeller used is a Hamilton Standard adjustable pitch type.

Four quickly detachable fittings form the engine mount attachment to the firewall. The use of specially designed rubber bushings prevents a large part of the engine and propeller vibration from being transmitted to the cabin.