awk to insert a files names

Shell Programming and Scripting
1

I have a couple of text files
which are named as

1.txt
2.txt
1000.txt
11.txt

I want to cat/merge the contents of each text file into one single file like this

1
content of 1.txt
2
content of 2.txt
1000
content of 1000.txt
11
content of 11.txt

I can merge the files and insert a space between the contents of the files using the awk

awk 'FNR==1{print ""}1' *.txt

But would like to know how to modify the above awk command to include the file names (numbers) at the space between merged files.

Please let me know

2 
awk 'FNR==1{print FILENAME} 1' *.txt

Try that

3 
$ more *.txt > outfile