We think it is some type of lily. It blooms in late summer and has waxy leaves. I transplanted some of these large bulbs from southwest Arkansas to the hills of northwest Arkansas. They are very hardy and like sandy soils with good drainage. The plant in question is a crinum lily bulb.
Crinum s are old-fashioned southern plants.
The foliage is not especially attractive, but the blooms are worth the foliage. The plant is in the amaryllis family and produces large, fragrant flowers that are white or pink in color. Normally, I would consider central Arkansas almost their northern range. Be sure to add extra protection from cold, winter weather since you have them in the northern part of the state. Give them plenty of moisture during their growing season, and full sun.