Which aircraft was developed to fly over enemy territory?

The U-2 and the SR-71 were specifically designed for reconnaissance and surveillance missions over enemy territory. These high-altitude, long-range aircraft were developed during the Cold War era to gather intelligence without being easily detected or intercepted by enemy forces. The U-2 is known for its ability to fly at extremely high altitudes, allowing it to photograph enemy landscapes and facilities with high resolution, while the SR-71, with its advanced speed and stealth capabilities, could penetrate deep into enemy airspace to collect critical data.

In contrast, the P-51, although an excellent fighter aircraft during World War II, was not primarily developed for flying over enemy territory for reconnaissance purposes; it was meant for air-to-air combat and ground support. The Boeing 727 is a commercial jetliner designed for passenger transport rather than military reconnaissance. Similarly, the Hawker Siddeley 125 is a business jet and is not intended for military use or operations over enemy lands. This distinction clarifies why the U-2 and SR-71 are the correct answer to the question regarding aircraft built specifically for intelligence-gathering missions in hostile environments.