Concatenate (Join) Cells Across Columns

I have a marco that works perfects to Concatenate (join) all of the cells in Column A. However, I want to seperately join all of the cells in each column. So I wanted to the Concatenate of Column B to go in B1, Column C to go in C1, etc (or until there is no data in a column). I think there is a way to change this into a loop, but I don't know how. Below is the macro I have now.

Sub ConcatenateAll()
Dim x As String, rng As Range, cel As Range
With ActiveSheet
Set rng = .Range("A2:A300")
For Each cel In rng
x = x & cel.Value
Next
.Range("A1").Value = x
End With

End Sub