The Runabout featured a distinctive wedge shape and took many styling cues from contemporary power-boat design.
Though the more extreme features of the Runabout such as the C pillar mounted headlights and the small wind-deflector windscreen were lost for the production car, many aesthetic features of the Autobianchi Runabout are readily identifiable on the X1/9.
The long flat bonnet (hood) with central indentation, the large front overhang, the wedge shape with prominent C pillar roll-over hoop and the car-length indented plimsoll-line all made the successful transition to the X1/9, giving it a highly distinctive appearance.
Designed around the all-new Fiat SOHC engine and transmission from the front wheel drive Fiat 128, the X1/9 relocated the transverse drive train and suspension assembly from the front of the 128 to the rear of the passenger cabin, directly in front of the rear axle, giving a mid-engined layout.
The layout located the fuel tank and spare wheel ahead of the engine, behind the driver and passenger seats respectively — optimizing the proportion of the car's weight within its wheelbase for more balanced handling and enabling cargo areas front and rear.
Once developed for production, the two-seater featured sharp-edged styling with a wedge shape, retractable headlights, an integrated front spoiler and a removable hard top roof panel (targa top).
The removable hardtop could be stored in the front boot; a second luggage compartment was provided at the rear of the car, accessible through a conventional boot lid.
Unlike Fiat's marketing nomenclature at the time which used a numerical system (e.g., 127, 128, 124, 131) denoting relative position in the model range, the X1/9 retained its prototype code as its marketing name.
Fiat's prototype coding used X0 for engines, X1 for passenger vehicles and X2 for commercial vehicles.
The X1/9 was thus the ninth passenger car developed using the nomenclature.
Originally slated for the November 1972 Turin Motor Show, the X1/9's launch was delayed until after the show to avoid upstaging the new Fiat 126 citycar.
Press test drives were held at the end of November 1972, on the Sicilian Madonie roads home to the Targa Florio road race. Small wrap-around steel bumpers with large rubber blocks; chrome trim rear fascia; oval holes in rear lower valence; manual choke (operating knob located between the seats); no access panel to distributor from spare tire well.