The Republic-Ford JB-2, also known as the Thunderbug, KGW and LTV-N-2 Loon, was an American copy of the German V-1 flying bomb. Developed in 1944, and planned to be used in the Allied invasion of Japan (Operation Downfall), the JB-2 was never used in combat. It was the most successful of the United States Army Air Forces Jet Bomb (JB) projects (JB-1 through JB-10) during World War II. Postwar, the JB-2 played a significant role in the development of more advanced surface-to-surface tactical missile systems such as the MGM-1 Matador and later MGM-13 Mace.

The United States had known of the existence of a new German secret weapon since 22 August 1942 when a Danish naval officer discovered an early test version of the V-1 that had crashed on the island of Bornholm, in the Baltic Sea between Germany and Sweden, roughly 120 kilometers (75 miles) northeast of the V-1 test launch ramp at the Peenemünde Army Research Center, on Germany's Usedom Island. A photograph and a detailed sketch of the V-1 test unit, the Fieseler Fi 103 V83 (Versuchs-83, the eighty-third prototype airframe) was sent to Britain. This led to months of intelligence-gathering and intelligence-sifting which traced the weapon to the airfield at Peenemünde, on Germany's Baltic Coast, the top-secret German missile test and development site.

As more intelligence data was obtained through aerial photography and sources inside Germany, the United States decided to develop a jet-powered flying bomb in 1943. The United States Army Air Forces gave Northrop Aircraft a contract in July 1944 to develop the JB-1 (Jet Bomb 1) turbojet-powered flying bomb under project MX-543. Northrop designed a flying-wing aircraft with two General Electric B1 turbojets in the center section, and two 900 kg (2000 lb) general-purpose bombs in enclosed "bomb containers" in the wing roots. To test the aerodynamics of the design, one JB-1 was completed as a manned unpowered glider, which was first flown in August 1944.

Motivated by intelligence information supplied by the United Kingdom, Project MX-544 to create an American copy of the V-1 was initiated. In July 1944, three weeks after German V-1 "Buzz Bombs" first struck England on 12 and 13 June, American engineers at Wright Field fired a working copy of the German Argus As 014 pulse-jet engine, "reverse-engineered" from crashed German V-1s that were brought to the United States from England for analysis. The reverse engineering provided the design of America's first mass-produced guided missile, the JB-2. The matter of guided missiles became a jurisdictional issue between Army ground Forces and the Army Air Forces. The dispute was decided in favor of the AAF as the JB-2 had wings and flew. It was decided that the JB-2 "should be placed in combat, needed or not, as an insurance measure for AAF claims.

By 8 September, the first of thirteen complete JB-2s, reverse engineered from the material received at Wright Field in July was assembled at Republic Aviation. The United States JB-2 was different from the German V-1 only slightly in dimensions. The wing span was only 2+1⁄2 inches (6.4 cm) wider and the length was extended less than 2 feet (61 cm). The difference gave the JB-2 60.7 square feet (5.64 m²) of wing area versus 55 square feet (5.1 m²) for the V-1. One of the few visible differences between the JB-2 and the V-1 was the shape of the forward pulsejet support pylon — the original V-1 had its support pylon slightly swept back at nearly the same angle on both its leading and trailing edges, while the JB-2's pylon had a vertical leading edge and sharply swept-forward trailing edge. A similar, completely coincidental re-shaping, but with a much broader chord, was used for the same airframe component of the manned Fieseler Fi 103R Reichenberg, original V-1 ordnance development. Initial testing of the first batch of prototype missiles resulted in a decision to change from the preset guidance of the V-1 to a command guidance system utilizing a radar beacon in the missile and radio command guidance from a tracking SCR-584 radar.

With its Ford-produced PJ31 pulsejet powerplant, the JB-2 was one of the first attempts at a powered cruise missile for potential usage in America's arsenal. Early launches of JB-1, JB-2, and JB-10 missiles were from a site along the coast on Santa Rosa Island in what is now Sandestin, Florida. The land necessary was leased at $1 a year for the duration of the war. The test location was 22 miles (35 km) east of Eglin Army Air Field designated Range 64 (aka C-64) 30°12′9″N 085°50′59″W / 30.20250°N 85.84972°W / 30.20250; -85.84972 (Range 64) (30°23′54″N 086°41′33″W / 30.39833°N 86.69250°W / 30.39833; -86.69250) and involved 2+1⁄2 miles (4.0 km) of the island’s shore line as well as extending 150 miles (240 km) off shore. Range 64 was a self-sufficient facility with not only launch and missile preparation facilities, but included barracks, mess hall and administrative facilities as well. Later a Training Unit P had to be located 1+1⁄2 miles (2.4 km) east of Range 64 to house men developing the techniques needed for field operations in combat while being trained to field the JB-2. Ultimately nine launch ramps of varying length and method of propulsion were built at Range 64. While copying the V-1 proved easy, inventing a means of launching proved very difficult. The first launch of a JB-2 from Range 64 was on October 12, 1944 by the 1st Proving Ground Group. A total of 233 JB-2s were launched from Range 64 between October 1944 and October 1945. All further tests at Range 64 were cancelled in November 1945 and orders were issued to deconstruct, remove or abandon everything by December 12, 1945. After the war ended the land leased for a single dollar was returned to the Coffeen family. The site of Range 64 is now the Coffeen Nature Preserve, Four Mile Village and Topsail Hill Preserve State Park in Sandestin, FL.

As the Heinkel He 111H-22 had done late in the war for the Luftwaffe in offensive air-launches of V-1s against the Allies, JB-2s were launched from MB-17G 44-85815 based at Hurlburt Field beginning in early 1945 and continued after the war ended.

In addition to Range 64, a detachment of the Special Weapons Branch, Wright Field, Ohio, arrived at Wendover Field, Utah, in 1944 with the mission of evaluating captured and experimental systems, including the JB-2. Testing was from a launch structure just south of Wendover's technical site. Unlike the missiles fired at Range 64 the ones fired at Wendover were armed and fired in the direction of a distant mountain range where their destruction by ground impact was assured. The launch area is visible in aerial imagery (40°41′53″N 114°02′29″W / 40.69806°N 114.04139°W / 40.69806; -114.04139). Parts of crashed JB-2s are occasionally found by Wendover Airport personnel.