Abstract: Systems, assemblies, and methods are described which relate to drilling a lateral borehole from a primary wellbore. A primary wellbore may have a casing therein. A window may be milled in the casing using a casing bit. The casing bit may pass through the window and into the surrounding formation. A protective sheath coupled to the casing bit may also extend through the window. The casing bit and protective sheath may remain in place while a drill bit slides through the protective sheath. The drill bit may drill through the casing bit and drill the formation to extend the lateral borehole. An anchor may be used to secure the protective sheath and/or casing bit in place. The drill bit may be included in a single trip assembly with the casing bit and protective sheath.