The Arlington County Board has given unanimous approval to Amazon’s plans to build a unique, helix-shaped tower as the centerpiece of its emerging second headquarters in northern Virginia. Amazon announced plans last year for the 350-foot tower. It will support a second headquarters for Amazon that is expected to welcome more than 25,000 workers when it’s complete. The helix stands out among several office towers granted approval in the redevelopment plans. The spiral design features a walkable ramp wrapping around the building with trees and greenery planted to resemble a mountain hike. The County Board approved the project at its Saturday meeting.