William C. Norris, founder and chairman of Control Data Corp. and a pioneer of the computer industry, will announce his retirement Friday after 28 years at the top of the Minneapolis-based computer and financial services company.

Norris' departure comes as Control Data is struggling to recover from the worst year in its history. Deep cutbacks in its troubled computer-peripherals business and other reverses produced a $255.6 million loss in the third quarter of 1985, and the company is in default on more than $300 million in short-term loans and credits.

As the bad news mounted, so did pressures in the financial community for Norris' departure. Sources close to the company said his decision to resign was influenced by his belief that the worst is over for the company and the time right for a transition. Norris, who turns 75 in July, had been expected to retire some time this year. He will remain a consultant and a Control Data director.

Control Data's board will turn full control over to Robert Price, 54, Control Data's president and chief executive officer, sources said. Price, a 21-year-veteran of Control Data, has been Norris' heir apparent since 1980.