In 1941, P-40D Ser No 40-360 was fitted with a 1300 hp British-built Rolls-Royce Merlin 28 engine with a single-stage two-speed supercharger. It flew for the first time on June 30, 1941. This experimental P-40D could be distinguished from the stock P-40E by the absence of the top-mounted carburetor air scoop. The Merlin engine did much to overcome the limitations imposed by the Allison, and a total of 1311 examples powered by the American-made version of the Merlin that was built by the Packard Motor Car Company were ordered under the designation P-40F.
The P-40F and later versions were known by the name *Warhawk* in US service.
The first 699 planes of the P-40F series had no dash numbers, since the production block designation system was not yet in effect. The dash numbers were first used with the P-40F-5-CU model, which introduced a fuselage elongated from 31 feet 2 inches to 33 feet 4 inches in order to improve directional stability. This longer fuselage was retained in all later P-40 versions. The P-40F-10-CUs had manual instead of electrically-operated cowl flap controls. The P-40F-15-CUs had winterizing equipment, and the P-40F-20-CUs had a revised oxygen flow system for the pilot. A radio mast was fitted to late production P-40Fs.
The P-40F was powered by a Packard-built Merlin V-1650-1 twelve-cylinder Vee liquid-cooled engine rated at 1300 hp for takeoff and 1120 hp at 18,500 feet. Maximum speed was 320 mph at 5000 feet, 340 mph at 10,000 feet, 352 mph at 15,000 feet, and 364 mph at 20,000 feet. An altitude of 10,000 feet could be attained in 4.5 minutes, and an altitude of 20,000 feet could be reached in 11.6 minutes. Maximum range was 700 miles at 20,000 feet (clean), 875 miles (one 43 Imp gal drop tank), and 1500 miles (141.5 Imp gal drop tank). Service ceiling was 34,400 feet. Weights were 6590 pounds empty, 8500 pounds normal loaded, and 9350 pounds maximum. Dimensions were 37 feet 4 inches wingspan, 33 feet 4 inches length (P-40F-5-CU and later), 10 feet 7 inches high, 236 square feet wing area. Armament consisted of six 0.50-inch machine guns in the wings.
One hundred and fifty P-40Fs were supplied to the RAF under Lend-Lease. The RAF assigned them the name Kittyhawk II. The Kittyhawk IIs were offset from USAAF allocations 41-13697/14599. RAF serials were FL219/448. Unfortunately, P-40Ls were also mixed in with this lot with no mark distinctions, so it is impossible to tell which planes were Fs and which were Ls by merely looking at the RAF serial number. In the event, very few of these aircraft actually served with the RAF. FL273 and FL369-448 were returned to the USAAF for use in North Africa in 1942/43. FL230/232, 235, 236, 239/240 were lost at sea before reaching the RAF. FL263, 270, 276, 280, 383, 305, and 307 were handed over to the Free French, who operated them in North Africa. 100 were transferred to the USSR.
The designation YP-40F was unofficially assigned to P-40F Ser No 41-13602 used for experimental tests of the cooling system and the tail rudder. The coolant system was moved aft in several different configurations, including a mounting fitted inside a thickened wing-root section.
USAAF serials of the P-40F were as follows:
41-13600/13695 Curtiss P-40F Warhawk 41-13696 Curtiss P-40F Warhawk (order cancelled) 41-13697/14299 Curtiss P-40F Warhawk 41-14300/14422 Curtiss P-40F-5-CU Warhawk 41-14423/14599 Curtiss P-40F-10-CU Warhawk 41-19733/19932 Curtiss P-40F-15-CU Warhawk 41-19933/20044 Curtiss P-40F-20-CU Warhawk
A number of P-40Fs were selected at random, withdrawn from operational service, and fitted with Allison V-1610-81 in place of their original Merlins. These planes were intended for training duties. These were redesignated P-40R-1. Similar conversions from the P-40L were designated P-40R-2. Army records report that over 600 such conversions were made, but only 70 such conversions can be confirmed by serial number