Hammel also said before Sunday's game that he felt fine after the throwing session.
Showalter said that if Hammel got through the innings with a lower number of pitches at Bowie, they'll have him go to the bullpen and throw some more.
"We'd like to get him to 45 [pitches]," Showalter said.
Hammel's been on the disabled list since July 29 with a right flexor mass strain. He'd been struggling throughout the summer and lost his last six decisions to drop to 7-8.
The right-hander hasn't won since beating the Nationals on May 27.
Jeff Seidel is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.