The "Speedmaster" name was chosen for its tachymeter scale bezel and following the Omega convention used for other models like the Seamaster - initially the Speedmaster was part of the Seamaster line - and the Railmaster.

 

This first Speedmaster model - reference CK 2915 also known as the "Broad Arrow"- was designed by the Swiss Claude Baillod and was already featuring some of the hallmarks of the model: the triple-register chronograph layout, the high-contrast index markers, and the domed Plexiglas crystal. The dial was an example of perfect balance and proportions. The model had straight lugs, broad arrow hands and the bezel was in steel with engraved black print. The case diameter was 39 mm.

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In 1959, a second Speedmaster version - the reference CK 2998 - was released with alpha hands and a black aluminium bezel to improve readability. The case diameter was expanded from 39mm to 40mm and, for the first time, Omega added the so called O-ring gasket around the push buttons to improve water resistance.

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Finally, with reference ST 105.002 in 1962 and reference ST 105.003 in 1963, Omega introduced the Speedmaster's typical straight baton hands. That same year - with reference ST 105.012 - a 42 mm asymmetrical case, adding protection to the chronograph pushers and crown, was released. This is the case that remains, little changed, in production today.

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Source:timeandwatches.com