Only two F-16XLs were ever made. They entered the Air Force's Enhanced Tactical Fighter competition to select a replacement for the F-111 Aardvark.
Aviation enthusiasts tend to recognize the F-16 Fighting Falcon on sight. The jet cuts a distinctive figure with compact proportions, a bubble-canopy, and one engine -- with the air intake located dead center and horizontal stabilizers forming a symmetrical angle. The airframe's ubiquity helps, too, with over 4,600 F-16s produced since the 1970s and over 2,600 still in service today (by comparison only 187 F-22 Raptors were produced).
But there was one F-16 variant, a little-known experiment, that few would recognize: the F-16XL.
Only two F-16XLs were ever made. In 1981, the fighter entered the Air Force's Enhanced Tactical Fighter (ETF) competition to select a replacement for the F-111 Aardvark. The F-16XL lost the bid, which is why you've probably never heard of the thing (the F-15 Eagle, which you probably have heard of, ultimately won the ETF competition, joined the U.S. Air Force, and has since been mass-produced).
The F-16XL looks similar to the standard F-16, like some sort of cousin, perhaps, with designers using the standard model as a baseline for the XL-variant. The two airframes are different, of course, most notably in regard to wing shape; the F-16XL features a delta design. The most expert observers might catch another subtle difference between the two F-16s; the F-16XL was 56 inches longer -- thanks to two sections added to the joints of the main fuselage sub-assemblies. Also, the F-16XL's tail was angled 3.16 degrees upward and the ventral fins were removed.
The original F-16 was relatively new and had performed admirably, so it may seem odd that the airframe was modified so drastically. But the delta wing design improved the lift-to-drag ratio during supersonic flight by 25 percent relative to the standard F-16. Test pilots reported that the F-16XL handled quite well at both low and high speeds.
Also, the larger, longer F-16XL was able to carry more fuel and more weapons. The enlarged wing allowed for the new F-16 to store 65 percent more fuel, which resulted in a 50 percent further range. And the massive delta-wing allowed for more external hardpoints and hence larger weapons payloads. The result was a jet that could deploy more weapons than its predecessors and could go further to deploy said weapons. And that marks a significant practical improvement over the F-16.
The F-16XL was originally a part of the General Dynamics experimental program known as SCAMP, or Supersonic Cruise and Maneuver Prototype. SCAMP tested a variety of wing types to find their desired characteristics. Ultimately, the delta-wing design was selected thanks to the aforementioned lift-to-drag ratio.
When the Air Force selected the F-15 as winner of the ETF competition, General Dynamics donated the two existing F-16XLs to NASA, which put the delta-winged jets to good use; the F-16XLs were flown, for research purposes, until 2009 -- nearly thirty years after first flying.
After being retired, the F-16XLs were transferred to Edwards Air Force Base, where they were stored, and where they have mostly faded into the annals of aviation mythology.