The coal strike may have affected the U.S. economy, but it has not affected the fortunes of the National Bank of Washington. Three quarters of the bank's common stock is owned by the United Mine Workers.

During the first two months of this year, NBW's net income was substanially ahead of the same period last year, Chairman Donald D. Notman said at yesterday's annual meeting. That gain follows a record year during which earnings rose 15.6 percent to $5 million ($4.55 a share) from $4.3 million ($3.95) in 1976.

"The bank is not for sale," President Walton W. Sanderson quoted UMW officials as declaring. The union has not sold nay stock during the strike, and the value of NBW stock increased slightly these past months from $33 to $36. Union balances in the bank have remained constant.

Three UMW officials, including President Arnold Miller, were elected to the bank's board by its stockholders. The bank's directors had already elected them in January to fill vacancies. Besides Miller, the new directors are UMW Vice President Sam Church, Jr., and UMW secretary-treasurer Williard Esselstyn.

In its annual report, NBW noted that loans and other transactions involving principal stockholders and directors/officers were made on "substantially" the same terms as to other customers. The amount of outstanding loans to directors and senior officers on Dec. 31, was $6.5 million, down from $7.8 million the previous year.The total of outstanding loans amounted to $395.9 million last year.