For the second time in a month a foreign bank has purchased a controlling interest in a major U.S. bank in one of the largest transactions in banking history.

Under a tentative agreement announced yesterday, the National Westminster Bank, Ltd., the second largest bank in Britain, will pay $300 million to acquire 75 percent of the New York-based National Bank of North America, the 40th largest U.S. bank as of the end of last year.

In early April, Hongkong & Shanghai Banking Corp, agreed to buy a controlling interest in the Bufflo-based Marine Midland Bank.

C.I.T. Financial Corp. has owned NBNA since 1965, when it purchased what was then called the Meadowbrook National Bank.

According to C.I.T. Vice President for Finance Richard Lund, National Westminister sought out C.I.T. for the purchase.

"They made an attractive offer," Lund said. "We saw the opportunity to capitalize on the situation. We will realize a nice gain - a profit in the $20 million range. We will have the opportunity to redeploy $300 million and we will still have a significant 25 percent interest in the bank."

Lund said, "We worked out a set of ground rules, and they met them. And, we think that the bank can improve its profitability far faster under National Westminister."

One of the main attractions of the sale, Lund said, was that it would rid C.I.T. of the constraints that had been placed on it as a bank holding company by banking regulatory agencies.

"When we bought the bank in 1965, there were no such things as one-bank holding companies," he said. "In 1970 the one-bank holding company was brought into existence, and we became heavily regulated. We could not made any acquisitions in the manufacturing or insurance areas, and any in the finance area had to go through the long process of Federal Reserve approval."

He would not discuss what acquisitions C.I.T. may have in mind for the future. "We still have up to eight months before this sale is finally worked out."

C.I.T. has interests in financing, insurance and manufacturing sectors. It will hold the option under the proposed transaction to sell all or part of its remaining interest in National Bank to the British at the initial per share purchase price.

The proposed sale would be one of the largest takeovers in U.S. banking history. It is still subject to the approval of the directors of both banks, as well as British and U.S. authorities.

National Bank has 141 offices in the New York metropolitan area, with total assets of $3.8 billion. Last year, it showed profits of $11.26 million.

National Westminster Bank is ten times the size of its acquisition, boasting assets of $36.6 billion. It currently has three offices in the U.S. specializing in international corporate finance.