During the late 1940s and early 1950s many aircraft manufacturers were conducting studies on the feasibility of converting existing piston-powered transports to turboprop power. The use of turboprops was seen as a low-risk advance that combined proven airframes with higher performance engines without sacrificing fuel economy, one of the weaknesses of jet engines of the day. With the Boeing C-97/KC-97 Stratofreighter in service with the USAF at the time, Boeing had pitched to the USAF several times a turboprop-powered Stratofreighter. All were under the same Model 367 number and at one point in 1953 Boeing went as far to built a partial mockup of the proposed Model 367-41.

The USAF, however, showed little interest in Boeing's proposals but in 1955, decided to investigate further the concept of a turboprop-powered C-97/KC-97 by commissioning Boeing to convert two aircraft (52-2693 and 52-2672, both KC-97Gs) to turboprop power. Pratt & Whitney YT34 turoprop engines (which would later be used on the Douglas C-133 Cargomaster) delivering 5,700 horsepower were substituted for the four R-4360 radial engines. For a brief time the USAF considered redesignating these two Stratofreighters as C-137, but ended up assigning them the designation YC-97J (ironically the C-137 got used for the Boeing 707s used by the military, itself a development of the Model 367-80 prototype).

The conversion to turboprop power shaved nearly 5,0000 lbs off the aircraft's weight as the YT34s were much lighter but more powerful. The first flight was made on 19 April 1955 and the YC-97J demonstrated significant improvements in overall performance. The top speed was 417 mph compared to 375 mph for a regular Stratofreighter and the YC-97J took only 14 minutes to reach 20,000 feet whereas the regular Stratofreighter took 50 minutes!

Both aircraft were flown in regular transport duties as well as trials work by the USAF until 1964, but by the time both aircraft had flown, Boeing and the USAF were shifting their efforts to developing the KC-135 Stratotanker and its even greater potential than the YC-97Js.

The first YC-97J, 52-2693, upon retirement in 1964 was used to provide parts and sections for the prototype Aero Spacelines B-377SG Super Guppy.

Source: International Air Power Review, Volume 20. AIRtime Publishing, 2006. "Warplane Classic: Boeing C/KC-97 Stratofreighter" by Bill Yenne, p128-129.

The first Air Force air-refueling tankers were KB-29 and KB-50s converted from bombers but by the 1950s the Boeing KC-97 became the standard air refueling tanker- this choice, however, was not without its difficulties particularly in refueling the B-47 Stratojet- the KC-97 wasn't able to reach the higher altitudes where the B-47 was most efficient and the B-47 had to descend to lower altitudes where its fuel burn was higher in order to link up with the KC-97. Furthermore, even with the KC-97's throttles "firewalled", the Stratojet had a tendency to stall during the link up and as a result, the refueling operation would have to carried out with the KC-97 in a shallow dive in order to gain speed above the stall speed of the Stratojet.

In 1953, the USAF trialled two B-47Bs outfitted with the British probe-and-drogue system developed by Flight Refueling, Ltd. One aircraft designated YB-47F and nicknamed "Pa" was fitted with nose mounted probe and the other aircraft, designated KB-47G and nicknamed "Ma", reeled out the hose-mounted drogue from its bomb bay. On 1 September 1953 the pair made the world's first all-jet air refueling.

Despite the higher speeds and ease of rendezvous, the fuel transfer rate with the system wasn't fast enough and in certain conditions was actually *less* than the burn rate of the six J47 engines of the receiver Stratojet. Only the flying boom used on the KC-97 was capable of the fuel transfer rates needed to give the Stratojets "legs" to reach distant targets. The interim solution was to fit the KB-50 and KC-97 tankers with auxillary jet engines to provide extra speed during refueling but the long term solution would arrive in the form of the Boeing KC-135 Stratotanker that had the speed and altitude performance to match SAC's bomber fleet.

Source: Boeing's B-47 Stratojet by Alwyn T. Lloyd. Speciality Press, 2005, p202.