Staples Inc. said Tuesday it will close more stores, implement leadership changes and invest in its online unit as the largest U.S. office supply chain looks for ways to cut down expenses and grow in a struggling economy.

The company said it plans to cut retail square footage in North America by about 15 percent by the end of fiscal 2015.

As part of that plan, Staples is accelerating the closure of approximately 15 U.S. stores, resulting in a pretax cash charge of around $35 million during the fourth quarter.

The company is also uniting its US retail and Staples.com businesses, and increasing investment in online and mobile units.

The cost savings plan is expected to generate annualized pretax cost savings of about $250 million by the end of fiscal year 2015.