Cumulative SUM formula

I want a cumulative formula that will return the running total for each value in column B.

For example, in the rows in column A:

1. =SUM(B$1:B1)
2. =SUM(B$1:B2)
3. =SUM(B$1:B3)
4. =SUM(B$1:B4)
5. etc.

I would like to get get this as an array formula so that I don’t have to drag the formula every time I add more data.

I haven't been able to solve this myself.