1965 - 1966

The 400GT (foreground) featured an uprated 3.9 litre engine. The Miura (background) became Lamborghini's first mid-engined two-seater.

In 1965, Gian Paolo Dallara made improvements to the Bizzarrini V12, increasing its displacement to 3.9 litres, and its power output to 320 hp (240 kW) at 6,500 rpm.^[29] The engine was first installed in the 400GT, essentially a 350GT with the larger engine. At the 1966 Geneva Auto Show, Lamborghini debuted the 400GT 2+2, a stretched revision of the 350GT/400GT that featured 2+2 seating and other minor updates. The 400GT 2+2, like its predecessors, was well received by the motoring press.^[30] The revenue from sales of the 2+2 allowed Lamborghini to increase the labour force at his factory to 170 employees, and expand services offered to Lamborghini customers.^[29]

During 1965, Dallara, Stanzani, and Wallace invested their personal time into the development of a prototype car that they envisioned as a road car with racing pedigree, capable of winning on the track as well as being driven on the road by enthusiasts.^[25] They hoped to sway Ferruccio Lamborghini away from the opinion that such a car would be too expensive and distract from the company's focus. When finally brought aboard, Lamborghini allowed his engineers to go ahead, deciding that the car, known as the P400, would be useful as a potential marketing tool, if nothing more.

The car's rolling chassis, featuring an unusual transversely mounted mid-engine layout, was displayed at the Turin Salon in 1965, impressing showgoers. A version with bodywork styled by Bertone was finished only days before its debut at the 1966 Geneva motor show. As had happened three years earlier at the debut of the 350GTV, an ill-fitting engine meant the prototype's engine bay was filled with ballast, and the hood kept locked.^[31] The favorable reaction to the P400 at Geneva led Lamborghini to slate the car for production by 1967, under the name Miura. The Miura's layout and styling would become the standard for mid-engine two-seat high-performance sports cars,^[32] a trend that continues today.

Lamborghini now had an offering that positioned the fledgling automaker as a leader in the world of supercars, while the 400GT was the sophisticated road car that Ferruccio Lamborghini had long desired to build. By end of 1966, the workforce at the Sant'Agata factory had expanded to 300, and enough deposits were made by prospective buyers to begin final development on the Miura in 1967. The first four cars produced were kept at the factory, where Bob Wallace continued to improve and refine the car. By December, 108 cars had been delivered.^[33]