In 1962, Piper began developing a six- to eight-seat twin-engined corporate and commuter transport aircraft under the project name Inca, at the request of company founder William T.

[3][4] It is a low-wing monoplane with a conventional tail, powered by two 310 hp (231 kW) Lycoming TIO-540-A turbocharged engines in "tiger shark" cowlings, a feature shared with the Twin Comanche and the PA-23 Aztec.

[4][5][6] As testing proceeded, two cabin windows were added to each fuselage side and the engines were moved further forward.

[6][7] The PA-31, named "Navajo" after the native American tribe, was certified by the FAA on 24 February 1966, again in mid-1966 with an increase in maximum takeoff weight (MTOW) from 6,200 to 6,500 lb (2,812 to 2,948 kg), and deliveries began in 1967.

[11] It was powered by 425 hp (317 kW) Lycoming TIGO-541-E engines, had a longer nose, fewer and smaller windows, 25 US gal (95 L) fuel tanks in the engine nacelles and a one-piece airstair cabin entry door instead of the split pair of doors.

[8][13] The Chieftain was powered by 350 hp (261 kW) Lycoming TIO-540 variants, with an opposite-rotation LTIO-540 on the right-hand wing, and MTOW was increased to 7,000 lb (3,175 kg).

[8] Deliveries started in 1973, after a delay due to a flood caused by Hurricane Agnes at Piper's factory in Lock Haven, Pennsylvania.

[15] The Navajo C/R had 325 hp (242 kW), lower rated versions of the Chieftain's counter-rotating engines.

[8] Up to eleven seats could be fitted, and baggage capacity was reduced from 700 to 600 lb (318 to 272 kg) maximum.

[19] The Chieftain's wings were strengthened, their span was 4 ft (1.2 m) wider and the fuel capacity was enlarged to 243 US gal (920 L).