As the 250 GTO was heavily derived from the earlier 250 GT Berlinetta SWB, Ferrari engineers constructed two 250 GTO prototypes in 1961 by converting existing chassis of this type.

The first prototype, designated in official photos as the 1961 Ferrari 250 GT Le Mans Berlinetta Sperimentale, was constructed from chassis 2643GT, originally a 1961 250 GT SWB.
It was built to competition specification, which included a reinforced chassis, a competition gearbox and a Tipo 168/61 3.0 L engine tuned to 300 bhp, equipped with dry sump lubrication and six Weber 38 DCN carburetors.
Pininfarina constructed a new lightweight aluminum alloy body for this prototype, which resembled that of the 400 Super America coupe.
2643GT was entered by Scuderia Ferrari in the 1961 24 hours of Le Mans, driven by Fernand Tavano and Giancarlo Baghetti.
Although they were running as high as 8th overall, they were forced to retire at 4:45 am on Sunday morning due to engine failure.
During the course of the race, Ferrari engineers gathered information about the performance of the car which was used to modify and improve it, including the addition of a rear spoiler.
During the Le Mans race, 2643 GT suffered from high-speed instability, possibly due to the front end design.
Following Le Mans, 2643 GT returned to the factory, where it was used for more testing.
The prototype raced again at the 1962 Daytona Continental 3 hours, where it placed 4th overall and 1st in the GT class driven by Stirling Moss.
Subsequently, it was sold to N.A.R.T. and a succession of private owners.

The second prototype was also constructed from a donor car, although sources disagree on the chassis number and type.
Several older sources mention the donor as a 1960 250 GT SWB, chassis 2053 GT.
Alternatively, other sources have claimed that a 250 GT Boano (0523GT) or a 1959 250 GT SWB (1791GT) was used as the donor car.
This prototype was created entirely by the Ferrari factory's racing department under the oversight of Giotto Bizzarrini, including the bodywork.
The original chassis was extensively modified, including relocation of the engine mounts lower and further back in the frame.
A competition-specification engine was fitted, including six Weber 38 DCN carburetors.
The bodywork seen on the second prototype in period photos was rough, unfinished aluminum.
The body's ungainly appearance lead the Ferrari team to nickname it "Il Mostro" (the Monster) and the press to call it "The Anteater."
Hammer marks, weld beads and bolted or riveted panels could be seen throughout, evidence of the continual modifications performed during factory testing in 1961.
Although the body was crudely formed, it displayed features that would be seen in the production 250 GTO, including the overall profile of a low hood and high rear, triple front air intakes, engine bay cooling slots in the front fenders and plexiglass-covered headlights.
The interior was hastily constructed and even more minimal than in the production 250 GTO, with scattered instrumentation and a bare aluminum dashboard.

The second prototype was tested at Monza in September 1961 by Stirling Moss.
Results were promising, as the prototype was able to lap the circuit faster than a 250 GT SWB.
However, the high speed stability issues seen during testing of the first prototype remained.
Shortly after this test, construction of the first production 250 GTOs began in late 1961 with chassis 3223 GT and 3387 GT.

As the prototype was no longer needed for testing, the experimental body was scrapped.
Regardless of the identity of the chassis, sources are in agreement that the second GTO prototype was either partially or entirely scrapped and is no longer extant in its 1961 form.
If 2053 GT was indeed the chassis, it was then given a 250 GT SWB style body and sold to Jacques Swaters.
2053 GT crashed during the 1962 Nürburgring 1000 km and was then rebodied by Carrozzeria Sports Cars.
Subsequently, 2053 GT was totally destroyed in an accident at the 1964 Spa 500km.
As the 250 GTO was heavily derived from the earlier 250 GT Berlinetta SWB, Ferrari engineers constructed two 250 GTO prototypes in 1961 by converting existing chassis of this type. 
The first prototype, designated in official photos as the 1961 Ferrari 250 GT Le Mans Berlinetta Sperimentale, was constructed from chassis 2643GT, originally a 1961 250 GT SWB. It was built to competition specification, which included a reinforced chassis, a competition gearbox and a Tipo 168/61 3.0 L engine tuned to 300 bhp, equipped with dry sump lubrication and six Weber 38 DCN carburetors. Pininfarina constructed a new lightweight aluminum alloy body for this prototype, which resembled that of the 400 Super America coupe. 2643GT was entered by Scuderia Ferrari in the 1961 24 hours of Le Mans, driven by Fernand Tavano and Giancarlo Baghetti. Although they were running as high as 8th overall, they were forced to retire at 4:45 am on Sunday morning due to engine failure. During the course of the race, Ferrari engineers gathered information about the performance of the car which was used to modify and improve it, including the addition of a rear spoiler. During the Le Mans race, 2643 GT suffered from high-speed instability, possibly due to the front end design. Following Le Mans, 2643 GT returned to the factory, where it was used for more testing. The prototype raced again at the 1962 Daytona Continental 3 hours, where it placed 4th overall and 1st in the GT class driven by Stirling Moss. Subsequently, it was sold to N.A.R.T. and a succession of private owners.
The second prototype was also constructed from a donor car, although sources disagree on the chassis number and type. Several older sources mention the donor as a 1960 250 GT SWB, chassis 2053 GT. Alternatively, other sources have claimed that a 250 GT Boano (0523GT) or a 1959 250 GT SWB (1791GT) was used as the donor car. This prototype was created entirely by the Ferrari factory's racing department under the oversight of Giotto Bizzarrini, including the bodywork. The original chassis was extensively modified, including relocation of the engine mounts lower and further back in the frame. A competition-specification engine was fitted, including six Weber 38 DCN carburetors. The bodywork seen on the second prototype in period photos was rough, unfinished aluminum. The body's ungainly appearance lead the Ferrari team to nickname it "Il Mostro" (the Monster) and the press to call it "The Anteater." Hammer marks, weld beads and bolted or riveted panels could be seen throughout, evidence of the continual modifications performed during factory testing in 1961. Although the body was crudely formed, it displayed features that would be seen in the production 250 GTO, including the overall profile of a low hood and high rear, triple front air intakes, engine bay cooling slots in the front fenders and plexiglass-covered headlights. The interior was hastily constructed and even more minimal than in the production 250 GTO, with scattered instrumentation and a bare aluminum dashboard.
The second prototype was tested at Monza in September 1961 by Stirling Moss. Results were promising, as the prototype was able to lap the circuit faster than a 250 GT SWB. However, the high speed stability issues seen during testing of the first prototype remained. Shortly after this test, construction of the first production 250 GTOs began in late 1961 with chassis 3223 GT and 3387 GT. 
As the prototype was no longer needed for testing, the experimental body was scrapped. Regardless of the identity of the chassis, sources are in agreement that the second GTO prototype was either partially or entirely scrapped and is no longer extant in its 1961 form. If 2053 GT was indeed the chassis, it was then given a 250 GT SWB style body and sold to Jacques Swaters. 2053 GT crashed during the 1962 Nürburgring 1000 km and was then rebodied by Carrozzeria Sports Cars. Subsequently, 2053 GT was totally destroyed in an accident at the 1964 Spa 500km.
Ferrari 250 GTO