LEWISTON, Maine — L.L. Bean hopes to give the boot to backlogs of its most iconic product. The Maine-based retailer is expanding production to keep up with demand for its leather-and-rubber “duck boot” with a new manufacturing center with another machine used to make the rubber soles. The company also plans to hire more than 100 additional production workers at two locations in Maine. “With the addition of the second molding machine here, we’ve significantly increased our capacity for Bean boot production,” said Steve Smith, the company’s president and CEO, told workers and guests Thursday. The old boot has done more than soldier on: Annual sales have grown from fewer than 100,000 pairs a decade ago to this year’s projection of 750,000 pairs.