The Pratt & Whitney X-1800 (later enlarged as the XH-2600) was an H-blockaircraft engine project developed between 1938 and 1940, which was cancelled with only one example being built.
The target date for series production was 1942. In 1940, however, performance on the test bench did not continue to improve, demonstrating a need for considerable additional development effort. Pratt & Whitney subsequently ended development of the X-1800 in October 1940, with only one built, to concentrate on radial engines.
Gunston, Bill (2006). World Encyclopedia of Aero Engines, 5th Edition. Phoenix Mill, Gloucestershire, England, UK: Sutton Publishing Limited. ISBN0-7509-4479-X.
Gunston, Bill (1999). The Development of Piston Aero Engines, 2nd Edition. Sparkford, Somerset, England, UK: Patrick Stephens, Haynes Publishing. ISBN0-7509-4478-1.