Can anyone identify this strange plane? It looks like they took wings from a bomber and put them on a fighter body... What was its purpose?
The Martin/General Dynamics RB-57F Canberra is a reconnaissance aircraft.
The RB-57F was derived from the Martin B-57 Canberra, a tactical bomber and reconnaissance aircraft. With its enlarged wings and more powerful engines, the RB-57F gained extreme altitude capabilities. It can climb up to 24,000 m (80,000 ft). It was developed and built for the US Air Force. The first flight was in June 1963.
403 B-57 have been produced, of which 21 have been converted to RB-57F. Some aircraft were operated by the air forces of Pakistan and Taiwan. Two RB-57F crashed on USA territory, and one aircraft was lost over the baltic sea. Only few aircraft have been salvaged. Most of the RB-57F ended up at the AMARC bone yard (source).
The particular aircraft on the question's picture with serial number 63-13291, was converted from a B-57 in 1964 and served the US Air Force till 1974, when it was put into storage at the AMARC bone yard.
picture source: NASA (https://acd-ext.gsfc.nasa.gov/People/Newman/wb57_pics.html)
picture source: NASA (https://airbornescience.nasa.gov/gallery/aircraft/WB-57_-_JSC)
Today there are still 3 planes (now called WB-57) in service for the NASA Airborne Science Program.
There is a Wikipedia article about this type.