through the propeller arc using gun-synchronizing gear. While the USAAC could block any sales it considered detrimental to the interests of the US, the NA-73 was considered to be a special case because it had been designed at the behest of the British. In September 1940, a further 300 NA-73s were ordered by the MAP. To ensure uninterrupted delivery, Colonel Oliver P. Echols arranged with the Anglo-French Purchasing Commission to deliver the aircraft and NAA gave two examples (41-038 and 41-039) to the USAAC for evaluation. The Allison engine in the Mustang I had a single-stage supercharger that caused power to drop off rapidly above . This made it unsuitable for use at the altitudes where combat was taking place in Europe. Allison’s attempts at developing a high-altitude engine were underfunded, but produced the V-1710-45, which featured a variable-speed auxiliary supercharger, and developed at . In November 1941, NAA studied the possibility of using it, but fitting its excessive length in the Mustang would require extensive airframe modifications and cause long production delays. In May 1942, following positive reports from the RAF on the Mustang I's performance below 15,000 ft, Ronald Harker, a test pilot for Rolls-Royce, suggested fitting a Merlin 61, as fitted to the Spitfire Mk IX. The Merlin 61 had a two-speed, two-stage, intercooled supercharger, designed by Stanley Hooker of Rolls-Royce. Both the Merlin 61 and V-1710-39 were capable of about war emergency power at relatively low altitude, but the Merlin developed at versus the Allison's at , delivering an increase in top speed from at ~ to an estimated at . Initial flights of what was known to Rolls-Royce as the Mustang Mk X were completed at Rolls-Royce's airfield at Hucknall in October 1942. At the same time, the possibility of combining the P-51 airframe with the US license-built Packard version of the Merlin engine was being explored on the other side of the Atlantic. In July 1942, a contract was let for two prototypes, briefly designated XP-78, but soon to become the XP-51B. Based on the Packard V-1650-3 duplicating the Merlin 61's performance, NAA estimated for the XP-78 a top speed of at , and a service ceiling of . The first flight of the XP-51B took place in November 1942, but the USAAF was so interested in the possibility that an initial contract for 400 aircraft was placed three months beforehand in August. The