The Fiat CR.40 was developed alongside the better known Fiat CR.32, and differed from that aircraft both in its choice of engine and the design of the upper wing.
The CR.32 had been developed from the CR.30, itself a new design that had replaced the family of aircraft that included the CR.1 and CR.20. The CR.40 was developed at the same time as the CR.32, but didn't make its maiden flight until 1934, a year after the CR.32, and at about the same time as the first production CR.32s entered service.
The CR.40 differed from the CR.32 in two ways. The CR.32 had a straight upper wing, connected to the fuselage by short struts. On the CR.40 the upper wing had a 'gull wing' shape, level for most of its length, but dropping down to connect directly to the fuselage in the centre. This greatly improved the pilot's forward view, allowing him to look across the top of the wing.
The second change was in the choice of engine. The CR.32 was powered by a Fiat A.30 RA bis inline-V engine, while the CR.40 was given an imported 525hp Bristol Mercury IV radial engine. As a result of its radial engine the CR.40 was shorter than the CR.32. The CR.40 had a top speed of 235mph, a very slight improvement on the 233 of the CR.32.
The CR.40 was followed during 1936 by a second prototype, the CR.40bis. This was powered by a 700hp Fiat A.59R radial engine, but could only reach 217mph. Work moved onto the CR.41, which reached the prototype stage before attention finally moved onto the CR.42.