The lack of modern conventional attack capability prompted calls for a specialized attack aircraft. On 7 June 1961, Secretary of Defense McNamara ordered the USAF to develop two tactical aircraft, one for the long-range strike and interdictor role, and the other focusing on the fighter-bomber mission. The former became the Tactical Fighter Experimental, or TFX, which emerged as the F-111, while the second was filled by a version of the U.S. Navy's F-4 Phantom II. While the Phantom went on to be one of the most successful fighter designs of the 1960s and proved to be a capable fighter-bomber, its lack of loiter time was a major problem, and to a lesser extent, its poor low-speed performance. It was also expensive to buy and operate, with a flyaway cost of $2 million in FY1965 ($ million today), and operational costs over $900 per hour ($ per hour today).