Bob Semple, at that time the Minister of Works, ordered this thing created, based on a D-9 tractor. It was top-heavy, very slow, under-armed and under-armoured, and if it had actually seen combat it's unlikely that it would have lasted more than a few minutes. Nevertheless, it was quite successful as a morale-building parade piece to keep up the spirits of the civilian population.
By 1944, the army had plentiful supplies of British and American tanks, and the "Bob Semple" was more of an embarrassment than anything else. They were disassembled and turned back into tractors, that being deemed their most useful possible contribution to the war effort.
I've long thought it a crying shame that there was no 15mm scale model of the Bob Semple tank to make its brave (but slow) way on to the wargames table. Now there is.
It's available from my Shapeways shop, starting at nineteen bucks for WSF.