Originally Posted by
physics
....
According to the United seatmap, there are two rows of only 2 seats at the window in rows 39-40. 39 is E+, but 40 is reg. economy. ...
UA list two 772 versions used for Hawaii
2 windows seat version for row 39-40
and
3 windows seat version for row 39-40 but 2 window side seats for Row 37
See the United or the thread's wiki
A problem is you will not know for sure until you board which aircraft you have. You will have a pretty good idea a few days before departure, but it can change even at that point (low chance but still possible)