Tanker's Tuesday:Medium Mk III tank

In 1926, the British War Office wanted to replace their existing Mark II tanks with a new design. In May the Royal Tank Corps Centre was asked for its opinion, which it submitted in July. One of the requirements was a weight limit of 15.5 tons, which led to the nickname "16-tonners". Other specifications included that it could transported by rail; a sufficient supply of lubrication oil to match the range of the tank (dictated by the fuel carried); a wireless set; a gun capable of defeating enemy armour at a range of at least a thousand yards; fuel tanks external to the main compartments and bottom armour sufficient to withstand heavy machine-gun fire when exposed while climbing a crest. Furthermore the machine should be as silent as possible, as with previous types the engine noise tended to incapacitate the crew.

 http://en.wikipedia.org/wiki/Medium_Mark_III