Yes, retirement income is taxable to North Dakota.
What about retirement from the Railroad Retirement Board?
North Dakota does allow for a subtraction on retirement income if the benefits are from the U.S. Railroad Retirement Board.
The taxable amount entered on the federal return for box 7a of form 1099-RRB, is automatically subtracted from your North Dakota taxable income.
See also Is my military pension/retirement income taxable to North Dakota?