The Austin-Healey 100 was developed by Donald Healey to be produced in-house by Healey's small car company in Warwick and based on Austin A90 Atlantic mechanicals. Healey built a single "Healey Hundred" for the 1952 London Motor Show, and the design impressed Leonard Lord, Managing Director of Austin so much that a deal was struck with Healey to build it in quantity at Austin's Longbridge factory. The car was renamed the Austin-Healey 100.
The "100" name comes from Donald Healey, who selected the name from the car's ability to reach 100 mph (160 km/h), as opposed to the Austin-Healey 3000, which is named for its 3000 cc engine.
Production Austin-Healey 100s were finished at Austin's Longbridge plant alongside the A90 and based on fully trimmed and painted body/chassis units produced by Jensen in West Bromwich — in an arrangement the two companies previously had explored with the Austin A40 Sports. The first 100s (series "BN1") were equipped with the same 90 bhp (67 kW) engines and manual transmission as the stock A90, but the transmission was modified to be a three-speed unit with overdrive on second and top.
Source: Wikipedia, 2011