CC Jordan on his web page fills us in on this model;
"Recent research appears to support Erik Shilling's strong argument that the AVG aircraft were very close to the P-40B configuration, which the British designated as the Tomahawk IIA. The history of the AVG fighters is nearly as interesting as the story of the AVG itself.
When the Chinese asked the U.S. for assistance, they asked for fighters and bombers. For political reasons, FDR would agree only to fighters, and U.S. law at the time allowed only for cash & carry sales to beligerents. So, a China based corporation was formed to purchase aircraft. However, virtually all U.S. production capacity was allocated for our own build up and existing contracts with friendly nations such as Britain. In order to free up some aircraft for China, the U.S. asked the Brits to exchange later model P-40Ds (Kittyhawk I) for currently ordered Tomahawk IIB fighters. The Brits agreed and 100 of the contracted Tomahawks were transferred to a Chinese contract.
Curtiss saw an opportunity to utilize stock of enternally sealed fuel tanks that had been used on the Tomahawk IIA. The Brits did not like the external sealing and specified internal sealing for future builds. This left Curtiss with over 100 sets of obsolete fuel tanks. This was their chance to use these, and they had already been written off. This would enhance profits. In addition, the Chinese contract, unlike that with the Brits, did not specify plumbing or shackles for an external fuel tank, so this was deleted from the Chinese aircraft. Again, this simplified production and increased the profit margin. The net result is that even though the 100 fighters carried Tomahawk IIB serial numbers, these fighters were very much like the IIA, except that they had IIB armor.
Allison was running at 100% capacity. Simply stated, there weren't any extra engines to be had. Every block and cylinder head was already allocated to an existing contract. But, wait a minute, there were plenty of rejected blocks, cylinder heads and such. Allison realized that most of the rejected engine components were usable if the various parts were hand matched and fitted. They set up a production line and began assembling these engines. Individual parts were reworked and carefully matched. The results of this procedure were engines built to very tight tolerances. Essentially, these were 'blueprinted' engines. Dyno tests revealed that they produced as much as 220 hp more than the production line V-1710-33s going into the RAF Tomahawks and USAAF P- 40C fighters. Allison had produced some very powerful and very expensive engines. Fortunately they were allowed to bury the extra cost into contracts for U.S. aircraft. These engines certainly account for the performance of the AVG's Tomahawks. In general terms, the AVG fighters could pull up to 370 mph in level flight, which is reasonable considering that these aircraft had 20% more power and less weight than the British Tomahawk IIB. Another fact not picked up on as significant by historians was the high rate of reduction gear failures in the AVG aircraft. This is easily explained when you realize that the older style reduction gear was rated for no more than 1,100 hp. With as much as 1,250 hp on tap, the reduction gearbox was over-stressed and frequently stripped gears. Later models, with 1,200 hp engines were fitted with a much stronger spur gear design that could handle up to 1,600 hp. This is the major reason that the nose is shorter from the P-40D onward.
Now that the Chinese had airframes and engines, they needed to purchase guns for the fighters. Once again, all production was allocated for existing contracts. Nonetheless, CAMCO (the China based front company) managed to purchase enough .50 caliber Brownings for all 100 Tomahawks. Finding .30 caliber guns (installed in the wings) was more of a problem. Eventually, the 100 Tomahawks were fitted with a mixture of guns. Some were fitted with 7.92mm caliber wing guns, others carried British specification guns in .303 caliber. Still others were armed with .30 U.S. caliber Brownings. This complicated logistics somewhat, but all three calibers were readily available, even 7.92mm, which was the standard for the Chinese Army.
Within the context of this knowledge, we can understand that the AVG fighters were a unique model. For that reason, Curtiss gave them their own special designation. Originally contracted for as the Curtiss H81-2A, these fighters were designated as the H81-3A. Many historians and authors have confused the various Curtiss designations, or figured that these were Tomahawk IIB aircraft based upon the serial numbers. We now know that these were a special model. It should also be noted that the serial numbers were assigned months before actual manufacturing began.
So, a unique group of fighter pilots flew an equally unique version of the Curtiss H81/Tomahawk/P-40."
http://www.geocities.com/pentagon/quarters/9485/P-40C.html Westy
