The creation of the Fiat 850 started at the end of the 1950s when Fiat management realized that there would be a demand for a car that was somewhat larger than the Fiat 600 and a bit smaller than the Fiat 1100. Dante Giacosa and his team are commissioned to design this successor.

At that time, Giacosa was busy with the further development of the Fiat 500, 600, 1100, 1800 and 2100, but still found time to think about what the car could look like. He assigned numbers 119 and 122 (later to become the Simca 1000) to the project (later the project numbers also were to be used as type names: 124, 125 etc.).
When the final design was clear, it was given the model number 100G (this number also became the type number for the engine).

A few key features that were essential for the new model:

  • Cheap to build so that the selling price could remain low. Important to be able to sell the car throughout Italy because the south was still poorer than the north.
  • Reliable and cheap to use. After extensive testing, the 100G engine proved to be a reliable and economical design with low maintenance costs.
  • Excellent driving characteristics:
    - Manoeuvrable, allowing the car to be used both in the cramped cities and on the winding roads of Italy.
    - Good handling and braking
    - Easy to use
  • Satisfy the taste of the average Italian.

A first idea was this design; an enlarged version of the Fiat 600:

850 1960

Fortunately, this design was not developed further (among other things because this body shape had poor aerodynamics), but the pontoon shape of the larger Fiats such as the 1100 was used as a basis.
850 1960 2a

As for technology, a development of the 600 engine would be used in the same setup as this model.
The engine was enlarged from 60x56mm (633cc) to 65x63.5mm (843cc). To do this, of course, the distance between the cylinders had to be increased.
And as we now know, the direction of rotation was also reversed to anti-clockwise (CCW), most probably because a bigger radiator was needed, like in Fiat 850's stepsister, the Simca 1000.

A bit of explanation*: For the Simca 1000 a new, all-synchromesh, gearbox was developped, in which the differential was offset to the right, so both driveshafts were of equal-length.
All this required the engine to run CCW in this model. To save development costs, the 850 was to use the same gearbox design, so a CCW derivative of the 600 engine had to be engineered.
*Thanks to Pedro Seixas Palma on Facebook (Fiat International Register group) for confirming this.

850 1960 technical design small
If you click on the image you will see a larger version.

This version includes the nose of the Autobianchi Bianchina's and the rear of the Fiat 1800:
850 1960 3a

This is already starting to look a bit like a Simca 1000, a design that Fiat used at its subsidiary Simca.
850 1960 6a

The management-approved design looked like this:
850 1960 approved

This would become the basis on which to work towards the final design.

In the meantime, testing was already being done with this "mule":

850 prototype

In 1961 it was decided to build the car in two-door body (because this was cheaper in production), although in 1962 the four-door version was picked up again:
850 1961

850 1962

The photos are from "Dante Giacosa: Forty Years of Design with Fiat". You can buy this biography on Ebay, among others

Testing continued, although the body still bears little resemblance to the Fiat 850 as we know it today.

850 prototype 1

850 prototype 2

850 prototype 3

Meanwhile, there was already much speculation in Italian car magazines about what Fiat was doing:
850 quattroruote850 voorspelling