The Senate Banking Committee yesterday overwhelmingly approved the nomination of industrialist G. William Miller to head the Federal Reserve Board, with only the committee's chairman, Sen. William Proxmire (D-Wis.), opposing the nomination.

The full Senate is expected to approve Miller's nomination today. The vote had been delayed more than five weeks while the Senate committee investigated whether Miller or the conglomerate he heads, Textron Corp., bribed the commander of the Iranian air force in order to get a $500 million helicopter contract in 1973.

Even as the banking committee was endorsing Miller on a 14-to-1 vote, the Securities and Exchange Commission went to court to try to force Textron to release documents the SEC had subpoenaed last month in connection with an investigation into questionable practices at the Providence-based company.

The SEC investigation was prompted by the Senate Banking Committee's probe of the Iranian payment by Textron's Bell Helicopter division to a company that allegedly was secretly controlled by the late Gen. Mohammed Khatemi. An exhaustive investigation by the banking committee staff did not turn up any evidence that Miller or other top Textron officials knew Khatemi secretly controlled the company, Air Taxi Inc.

In two separate appearances before the banking committee, Miller said that the $2.9 million payment was a legitimate termination agreement with Air Taxi, which had been Bell Helicopter's sales agent in Iran.

Textron refused to release the documents to the Securities and Exchange Commission until it could get a court order protecting their confidentiality.

Textron said that the SEC filed its court action "to permit Textron to seek a court order to protect documents containing confidential matter, such as information which should not be disclosed to competitors of its divisions."

Other companies under SEC investigation have used a similar procedure to keep documents they supplied the SEC from being made public. Among those firms are Lockheed and Boeing.

Textron said it has agreed to an expedited hearing in U.S. District Court here and will apply next Tuesday for an order protecting the confidentiality of its documents.

Because of the SEC investigation Proxmire had asked Miller to voluntarily withdraw his name, something Miller heatedly refused to do. Proxmire said that the investigation of Textron would leave a cloud over the Federal Reserve for the four to six months the SEC will take to complete it.

The five-week banking committee investigation upset many of the Democrats on the banking committee who complained that it was taking on the trappings of a fishing expedition. The committee was in near open revolt against its chairman expedition. The nority member, Edward Brooke (R-Mass.), last Tuesday.

Yesterday Proxmire admitted that he may have been to prosecutorial in the investigation -- noting wryly that his "immediate successor" was Joseph McCarthy -- but defended the investigation as thorough, necessary and completed as quickly as possible.

Proxmire said he did not know members of the committee felt the nomination was being delayed unduly and said it was his fault that members did not feel they could come to him with their thinking.

Proxmire -- who voted against Miller in part because of the Iranian payment but also because he feels the businessman is not qualified to head the Fed -- was given a unanimous vote of confidence by the committee before the vote on the Miller nomination.