Before you start with it here is what you’ll need in this guide.

1. iTunes 9
2. Pwnage Tool 3.1.4
3. Download Original iPhone OS 3.1.2 Firmware

How to Jailbreak iPhone 3G 3.1 Firmware:

1. Download and Install iTunes 9 from Apple site.
2. Make complete backup of your iPhone.
3. Download Original iPhone 3.1.2 Firmware using the links provided in this post.
4. Download Pwnage Tool 3.1.4 and select your device from devices list.
5. Now Pwnage tool will automatically detect original
   firmware file which you downloaded in Step 3, if it can’t then browse it manually.
6. Click Next and then Choose Yes and then on another dialogue box Choose no, if you aren’t using Contract iPhone. Choosing No will let you unlock your iPhone and use it with any SIM
7. Now Pwnage Tool 3.1.4 will start building custom IPSW file.
8. Once done, click Yes and then click Next.
9. Now Open iTunes 9 and choose your iPhone from the devices list on left side now press Alt/Option key and then click on Restore and browse the Custom made iPhone 3.1.2 firmware now choose Open.
10. Now relax back until iTunes updates your iPhone to 3.1.2 Firmware.

After completing the entire above steps your iPhone 3G 3.1.2

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This custom made 3.1 firmware comes with Apple logo and Cydia. So now follow the step below to jailbreak.

How to Jailbreak iPhone 3G Firmware 3.1

1. First Download the following links of
   custom iPhone 3.1
   Firmware
2. Now join all the downloaded parts using HJSplit and you will get one archive, uncompress that archive and it contains IPSW file of
   firmware 3.1 Custom iPhone 3G 3.1 Firmware Part 1
   Custom iPhone 3G 3.1 Firmware Part 2
   Custom iPhone 3G 3.1 Firmware Part 3
3. Now Install (if you haven’t already) and open
   iTunes 9
4. Connect you iPhone to the PC using data cable.
5. Choose your iPhone from devices menu on left.
6. Now click “Resotre” while holding Shift key and select the custom IPSW file which you downloaded in step 1
7. Now Sit back and relax until the installation process complete.
8. After Jailbreaking your iPhone 3G to 3.1 Firmware you can follow this guide to unlock you iPhone and use with any SIM/Carrier.

How to Jailbreak iPhone 2G Firmware 3.1

1. First Download the following links of
   custom iPhone 3.1
   Firmware
2. Now join all the downloaded parts using HJSplit and you will get one archive, uncompress that archive and it contains IPSW file of
   firmware 3.1 Custom iPhone 2G 3.1 Firmware Part 1
   Custom iPhone 2G 3.1 Firmware Part 2
   Custom iPhone 2G 3.1 Firmware Part 3
3. Now Install (if you haven’t already) and open
   iTunes 9
4. Connect you iPhone to the PC using
   data cable.
5. Choose your iPhone from devices menu on left.
6. Now click “Resotre” while holding Shift key and select the
   custom IPSW file which you downloaded in step 1
7. Now Sit back and relax until the installation process complete.

No ultrasn0w or Bootloaders needed for iPhone 2G.

Link Credits [Sizlopedia]